MM.DB and Mediamonkey.ini default file locations ( http://www.mediamonkey.com/forum/viewtopic.php?p=123329#p123329 ):

Windows XP: C:\Documents and Settings\<USERNAME>\Local Settings\Application Data\MediaMonkey
Windows Vista: C:\Users\<USERNAME>\AppData\Local\Mediamonkey
(Make sure to enable viewing of hidden files/folders)

Last.fm problems ( http://www.mediamonkey.com/forum/viewtopic.php?p=148972#p148972 ):
You should only use ONE of the options below. Using multiple plugins will result in your last.fm scrobbles being lost.

Option #1 - (Using last.fm client - I have tested this, it works - but you will be pestered with upgrade messages)
1. Download this: http://static.last.fm/plugins/ass/WinampPluginSetup_2.0.45.0.exe
2. Install to \Program Files\Mediamonkey\Plugins
3. When last.fm tells you to update your plugin, don't do it, the newest plugin doesn't work.
4. Scrobble!

Option #2 - (Old winamp standalone plugin - works but no support for foreign languages and seems to forget your login info periodically)
1. Download this: http://static.last.fm/plugins/audioscrobbler.wa.1.1.10.exe
2. Install to \Program Files\Mediamonkey\Plugins\
3. Choose 'General' option rather than 'Media library' option when installing.
4. Restart MM or start it, and go into Tools -> Options -> General Plug-ins to enter your login information
5. Scrobble!

Option #3 - (Native MM last.fm plugin - works very well but requires MediaMonkey 3.1.0.1217 or later!)
1. Download: http://www.mediamonkey.com/forum/viewtopic.php?f=6&t=34632
2. Double-click the downloaded file, and agree to install
3. Go to Tools > Options > Last.fm Service to configure your username and password