Just reading the FAQ for the MSN adcenter:
Microsoft adCenter is a Web-based program that can be used on any computer. You will need to make sure you have the following: o A working Internet connection. o Microsoft Windows 98 or later operating system installed. o Microsoft Internet Explorer 6.0 or later Web browser installed.and
C. Which browsers work with Microsoft adCenter? Microsoft Internet Explorer 6.0 or later. Microsoft adCenter is currently incompatible with Firefox, Safari and Internet Explorer 7 (beta) browsers. As Microsoft adCenter becomes compatible with additional browsers, we will update this site.So basically you're telling all Mac users to "get lost" or "we don't want your money"? I would rant about the Firefox users, but as they won't even allow their own browsers to access the site it might be a bit pointless ...
So, IE6 or later, except for IE7. Great, great.