I'm no guru, but it sounds like your security settings are too high for the Autodesk website. I also seem to recall that there's another site they use in addition to the Microsoft site to assist the webcast streaming. You may want to try adding all three to your Trusted Sites (Internet Explorer: Tools --> Internet options --> Security tab) and see where that gets you.
DISCLAIMER: I'm no guru and may in fact be talking through the hole in my head. But I'm not suggesting something I wouldn't try myself.
Hope that helps,
Trevor