is that all your code or is there more?
you need to make sure you set MsgrUIA to a value, this can be done in a plugin by setting the object that is passed in the initialise function.
otherwise you need a line that goes something like
code:
set MsgrUIA = New messengerapi.Messenger
although that might not exactly be it as i dont have any vb source on the computer im at right now.