To step through into the MM source, put a breakpoint in your code. When execution stops on the breakpoint, from the VS IDE choose Debug | Windows | Modules to see the modules your app has loaded.

Right click on say mere mortals framework.dll and choose reload symbols. Navigate the the program debug database (mine is at c:\program files\mere mortals .net framework 2003\mere mortals framework\Mere Mortals Framework.pdb). Choose that file and then you'll see in the modules window "Symbols Loaded" under information instead of "No Symbols Loaded". You can step through the code in mere mortals framework now.

Like I said, this is kind of a pain because you need to reload all the modules manually when you want to step through the code. This process needs to be repeated for all MM .dlls you want to step through.

Hope that's clear,
Mike