I have a Mac too and I had the same problem trying to figure it out.
There's a messenger program called aMSN that you can use to go on webcam and stuff with others who are using Windows Messenger.
http://www.amsn-project.net/
I wouldn't say it's the best messenger program, so I kept Microsoft Messenger on my computer as well as aMSN and I only use aMSN when a friend wants to do video chat with me.
I haven't found another program you can use to do this, but there are some programs you can install on your Mac that let you install Windows onto it, but I haven't done this because it seems like just too much work.
