I just installed MAT 1.2.1 as a plugin for Eclipse Indigo. After reviewing all the documentation, and even an online video, I cannot figure how to even start MAT. There is nothing in Eclipse menus or toolbars, and no executable anywhere that I could start directly. Sorry if this sounds like a newbie question, but sorry, it isn't.
You could try opening the Memory Analyzer perspective (if you haven't tried already).
With the Memory Analyzer feature installed, try Window -> Open Perspective -> Other... , then select Memory Analyzer and OK.
Once the MA perspecive is open, one should be able to see the Memory Analyzer specific Eclipse menus.