[Translation: In today's Shi'ur we find that Rebbi Yochanan ben Zakai promised his student, Rebbi Elazar ben Arach, that if he is a "Chacham and Mevin mi'Da'ato," he would expound before him.
The Gemara cites a Beraisa which relates that three people expounded the Ma'aseh Merkavah, but Rebbi Elazar ben Arach is not mentioned. The Gemara answers that the Beraisa mentions only those who expounded Ma'aseh Merkavah, and who had others expound it in front of them. Therefore, Rebbi Elazar ben Arach is not mentioned (since no one expounded it in front of him, even though he himself expounded it in front of someone else).
If Rebbi Elazar ben Arach is not mentioned because no one expounded in front of him, then Rebbi Yochanan ben Zakai did not keep his promise, when he said that he would expound it in front of Elazar ben Arach! How can this be?]
Rebbi Yochanan ben Zakai never promised such a thing outright to Rebbi Elazar ben Arach, and therefore he was not bound to expound in front of his student (and he indeed didn't, as Tosfos mentions).
(When Rebbi Yochanan ben Zakai said to his student, "Did I not teach you that we do not expound the Ma'aseh Merkavah to a single student unless he is a Chacham and Mevin m'Da'ato," he was stating the Halachah that appears in the Mishnah (11b), and he was not making any sort of promise to Rebbi Elazar ben Arach.)