Shoutbox

So, if you go to Plus! > Preferences > Plugins, it doesn't show up in the list? Try this:
Close Messenger.
Go to Start > Run, write "regedit" and press Enter.
Open the tree following this path:
  HKEY_CURRENT_USER
    Software
      Patchou
        MsgPlus2
Click the + next to MsgPlus2 and if you see a folder named "stuffplug-ng", right-click it and click Delete.

Now open the tree following this path:
  HKEY_CURRENT_USER
    Software
      Patchou
        MsgPlus2
          <your e-mail>
            Preferences
              Plugins
Click the + next to Plugins and if you see a folder named "stuffplug-ng.dll", right-click it and click Delete.
Restart Messenger, log-in into your account and go to Plus > Preferences > Plugins. Does it show up there now?

Important note:
What I described above is editing the registry. The registry is very important for Windows to be able to start and work correctly, so you should be always be very careful. If you follow my instructions exactly then nothing will happen, you'll just reset StuffPlug options.