Unfortunately it's a bit of a problem.
The Microsoft MSN Messenger program doesn't support voice or video on Macs.
There's a beta version of the next version (which does supposedly support voice/video) floating around the internet, but I haven't been able to get either to work with it - other people reportedly have.
aMSN supports video but not voice. The interface is awful but it gets the job done.
I've ended up convincing most of my friends with whom I videochat to install Skype as that works beautifully on Mac and PC.