Why did Moshe enter the Ohel Mo'ed with Aharon at this point?
Rashi #1: In order to teach him how to bring the Ketores. 1
Rashi #2, Rashbam and Targum Yonasan: To pray for the Heavenly fire (which is synonymous with the Shechinah) to descend to burn the Korbanos. 2
See Sifsei Chachamim.
When Aharon saw that, despite all the Korbanos he had brought, the Shechinah had not appeared, he became despondent, and convinced that Hashem was angry with him for his role in the Golden Calf, he accused his brother of embarrassing him. So Moshe went with him to the Ohel Mo'ed to pray, and the Shechinah came down in the form of the Heavenly fire.
What B'rachah did Moshe and Aharon now confer upon the people, which resulted in the Shechinah descending?
Although Targum Yonasan does not quote the actual words of Tehilim, he quotes a similar B'rachah.
Rashi writes that Moshe entered to teach Aharon how to bring the Ketores. How could he delay the Ketores so long? It is offered between blood of the Tamid and its limbs! And did he not offer Ketores during the seven days of the Milu'im?
Moshav Zekenim: He delayed it because he was waiting for fire to descend from Shamayim. And during the seven days of the Milu'im they offered only Korbanos Yachid, but not Olas Tamid, and not Ketores, for it is burned only with Olas Tzibur, and they take fire from the outer Mizbe'ach.