Try this, extract the file to your messenger plus! directory
(C:\Program Files\Messenger Plus!3)
It will run the exe with the uninstall commandline switch and activate the uninstaller.
If you cannot unzip the application download either
Winzip or
WinRAR
and then extract the file to your messenger plus directory.
(If you use winzip and just double click the winzip folder it will extract the file to C:\Unzipped\uninstall, you will need to manually copy the file to the location you have installed Messenger Plu!3 to.