You should have received your mail with the MCID after you passed your first exam. Subsequent exams do not generate an additional ID.
You can get in contact with your Regional Service Center and explain the problem. They should be able to resend your welcome email.
You can find a list of the RSCs in this page:
https://www.microsoft.com/learning/en-us/help.aspx
Warning: this link doesn't work from IE9, at least for me; I have to open it using Chrome instead.