Customers buying through the MPSA need access to the Microsoft Volume Licensing Center (MVLC) and there’s an updated (September 2014) guide.
The previous guide was written at the time of the Windows Azure Active Directory Preview Portal and so all the processes and screenshots are (nicely) updated for the new Azure Management Portal. There’s also a useful new table on MVLC roles and permissions on pages 19/20 and one on AAD admin roles and permissions on page 31.
If your job involves accessing or teaching people how to use MVLC then the instructions in Appendices A and B are worth a read. Get it here: http://bit.ly/1vIzfze.