Try going here and clicking the "live video" link. It'll tell you what plugin you need then. (I'm on a PC, so I'm not sure what the Macs use.)

There will be a link here and/or on the main site so you can watch the debate- I'm surprised it's not already up there. There's going to be a plain feed, and according to a friend who works down there, "Also a second stream with the people meter over the debate showing real-time reaction graph of a group of undecided democrats and republicans Texas voters."

Thanks, BP.