I'm not quite sure what password you have forgotten. If you have forgotten the Preferences password, you can do two things.
The simple solution is to uninstall Messenger Plus! without keeping your preferences. When you reinstall it Plus! will use the default settings again, which means no password.
The more difficult solution does not require you to uninstall and lose all your settings, but you have to do it very carefully. If you change the wrong thing, you might make your computer unusable.
- Open the Registry Editor (go to Start > Run and open regedit.exe).
- Go to the 'folder' HKEY_CURRENT_USER\Software\Patchou\MsgPlus2\<your email address for Messenger>\Preferences (click the Preferences folder)
- Double click 'UDataP' in the right side of the window and change the value to 0.
- Click OK and close the Registry Editor.
You should now be able to set the Preferences again. If this was not the password you were forgotten, please be more detailed