There is a possibility that you have another version of MySQL running... when you were working on the Mac's built in server, did you by chance make it so MySQL would try to start upon start up?
A couple of things to try...
Go into system preferences and click the "sharing" icon... be sure that Web Sharing is Unchecked.
The next thing to try is opening terminal, then type in the command: killall -9 mysqld
That would stop every instance of mySQL from running... then try and restart MAMP.
Let me know where you get from there.