Hi Nora,
The prereqs for the master credentials are as follows:
Excel Expert (example skills measured here:
http://www.microsoft.com/learning/en/us/Exam.aspx?ID=77-888&Locale=en-us#tab2 )
Word expert, PowerPoint and either Outlook or Access. From the latter choose one.
When you pass all these exams you will be Master. So there are no special exams to take to become a MOS Master just do the exams (in any order you like) to receive the credential with accompanied certificate :-)
Maurice
p.s. Excel Expert and Word Expert exams coming soon others already available
Maurice