1) One of the things which the Gemara in Menachos (29a) mentions that Moshe Rabeinu had difficulty with was the Menorah, which was one of the items in the Mishkan, so the Gemara does mention the Mishkan as one of the topics Moshe had difficulty with.

I think that the Rashi you are referring to in Parshas Terumah, Daniel, is in Shemos 25:31, where Rashi writes that Moshe Rabeinu was finding it difficult to understand how to make the Menorah, so Hash-m told him to throw the block of gold into the fire and the Menorah would be made automatically. However, as I mentioned above, the Menorah is mentioned in the Gemara in Menachos 29a as one of the things that Moshe found difficult.

The other Rashi in Chumash, about Moshe having difficulty, is in Shemos 39:33 (see below, #3). However, when one looks into this, it is not difficult to understand why this is not one of the things mentioned in Menachos 29a. The Torah says there that they brought the Mishkan, with all its prepared parts, to Moshe. Rashi explains that the reason they brought it to Moshe is that Bnei Yisrael were not able to erect the Mishkan. Rashi writes that Hash-m deliberately left the honor of erecting the Mishkan to Moshe, because he had not done any of the work in building the Mishkan.

The reason why nobody could set up the Mishkan was because the boards were so heavy. Moshe asked Hash-m, "How is it humanly possible to lift these boards?" Hash-m said to Moshe that he should move the boards and other pieces as if he wanted to erect the Mishkan, and Hash-m would miraculously make the heavy items stand up. It would look as if Moshe was constructing it, but it was actually all a Nes.

This is not similar to the other three examples mentioned in Menachos 29a. There, we learn that (a) Moshe did not know what the Menorah should look like and what was the proper way of making it; (b) Moshe did not understand exactly what the moon should look like at the beginning of the month, so that one could proclaim Rosh Chodesh; and (c) there were so many forbidden species of crawling creatures that Moshe did not understand how he could possibly remember all of the species. As an answer to these three difficulties of Moshe, (a) Hash-m told him to throw the gold into the fire and He would miraculously create the correctly shaped Menorah; (b) Hash-m would show Moshe exactly what the new moon should look; and (c) He showed Moshe all the forbidden kinds of Sheratzim.

In contrast, concerning erecting the Mishkan, it was not that Moshe did not know how to erect it, but rather it was phsically impossible to erect it. Hash-m answered that indeed it is physically impossible to lift up those heavy beams, but He would perform a miracle and make it look as if Moshe was lifting them.

2) The Midrash Rabah (Shemos 15:28) tells us that Moshe also had difficulty understanding how to make the Shemen ha'Mishchah until Hash-m showed him. The Radal there writes that the Gemara does not mention this because the word that the Midrash learns this from is "Zeh" -- that Hash-m showed it to him and said "this." However, the Gemara in Kerisus (5b) learns from the Gematriya of "Zeh" that the Shemen ha'Mishchah had to be 12 Lugin.

3) Regarding Rashi in Shemos 39:33, who writes that Moshe Rabeinu did not understand how a human being could set up the Mishkan until Hash-m told him to make himself look as if he was setting up and then it stood up on its own, one might also suggest as follows. The Gemara in Menachos does not mention this because this was a pure miracle, and was not something which Moshe did himself. It was done entirely by Hash-m.

Kol Tuv,

Dovid Bloom

1) The Sifsei Chachamim is on Rashi 39:33. However, in my edition of the Sifsei Chachamim, no mention of Tosfos is made. It must be pointed out that the Sifsei Chachamim is not a Rishon that we have a Mitzvah to be Meyashev. Rashi cites a Midrash Tanchuma, which is from Chazal, that Moshe merely looked as if he was erecting it, but really it all happened miraculously. I am not aware that there is a source in Chazal that disagrees with this.

2) However, we may suggest that the fact that Moshe Rabeinu was 10 Amos high helped it look less like a Nes. In reality, it was a Nes, but Hashem usually arranges that miracles happen in such a way that there is a possibility of understanding the phenomenon as a natural occurrence.

3) This is what happened here with Moshe. Even a 10-Amos giant would not have been capable of erecting the Mishkan without miraculous help, but there was still room for the sceptics to claim that the reason he managed it was because of his tremendous strength.

4) I looked in the source of the Sifsei Chachamim's words, Sefer Nachalas Yakov (Rav Yakov Solnik, whom I believe was a Talmid of the Rema and the Maharshal). One of the sources cited to suggest that Moshe may have erected the Mishkan in a natural way is the Gemara that states that Moshe spread out the cover on the Mishkan, which was 10 Amos high, and Moshe was also 10 Amos tall. One of the places where this is mentioned is Nedarim 38a. The Mefaresh there (DH Pirso) writes that the fact that Moshe spread it out proves that he was a "Gibor" and possessed a great body. I am not intending here to go into the question of whether this indeed proves that the erection of the Mishkan was done naturally and not by a miracle. I do not believe there is a proof from here. However, to answer to your original question, Daniel, I would say that according to those who argue that it was not a miracle, it is not difficult to understand why it is not mentioned as one of the things that Hashem showed to Moshe. This is because, on the contrary, according to this way of looking at it, it was a natural event which Moshe Rabeinu knew how to do on his own.

Dovid Bloom