unfortunately you have come across one area where the mac is lacking. msn for mac doens't let you use your webcam, nor will any 3rd party alternatives (bar maybe one, but that is FAR from perfect, or even very usable)
Apple do offer iChat AV for 10.3 users and above, which is incredible to use, being far superior quality and usablity than it's pc counterparts, but is crippled by only supporting the .mac or AIM protocols, and not msn.
if you have any AOL contacts, this is your only viable option, pretty much.