To become an Alumni you had to be a member of MTP during your studies. After completing your degree you automatically migrate into the Alumni status.
You were a member during the course of your studies not a MTP Alumni though you want to be? Than this is the way to benefit from the advantages offered by the alumni network:
It suffices to send an email with your name and your former regional office to the Alumni Administration and you get an application form or your membership status will be adjusted immediately.