Why wont my MSN messenger work (on my Mac computer??!?!??!?

I quit messenger and when i started it back up a window pops up and says "A newer version of MSN messenger for Macintosh is available. You must download the newer version in order to continue using MSN messenger. Would you like to do this now?" When i click yes Stuffit Exspander opens up and says that messenger does not appear to be compressed or encoded. It is advised that you obtain further information about the contents of this file. Please help i need to get back on!