As far as I remember from the website, when you install
Messenger Plus! Live, it disables old version of Messenger Plus!. If so, reinstall old version
www.msgpluslive.net/download/old
Also note that, you may encounter some problems if you sign in both of them at the same time. Also check out here
http://www.msgpluslive.net/help/faq/general/#mix-versions