I am an Erasmus / exchange student: how am I related to MSIAM?

You do not register in MSIAM. You are registered separately to each course, and belong to the separate mailing list of each course. You should warn the teacher of each course of your attendance, and the heads of M2 MSIAM of your choice. But they do not validate your choices, which are determined by your learning agreement. In case of changes in your list of courses, you should warn the heads of MSIAM, and the person in charge for exchange students at your university. The teachers transmit your grades to the relevant person at your university.