Open up the main contact list window for Windows Live Messenger, and go to the Plus! Preferences, Plus! > Preferences, now go to Scripts and press on "Hopper Live" and remove.
Now go to
http://www.msgpluslive.net/scripts/browse/index.php and download Hopper Live, make sure you
save the file, not just run it. Then start up Windows Live Messenger and install the file as usual.