In the past the answer has been "you don't", often coupled with enthusiastic statements about the mbone being in full production, and tunnels no longer being necessary.
I contacted my ISP last week about getting multicast routing configured and they spent days trying to get it working. They finally called their equipment vendor, who explained to them that they need someone to route to them, as well, which was news to them. I'm now getting an mbone feed through work (I'm a telecommuter so it wasn't automatic).
You can sometimes get a multicast tunnel terminated by a friendly university
department, if you've got such thing, but they're not as happy about letting
you pump a lot of traffic through them as they used to be.
BTW, the only multicast stream working during last night's plenary was the MPEG-1 one, and because of the high bit rate it was very lossy. The other, lower bitrate streams weren't working and the guy running the service kind of blew me off (and I quote: "watch the video in a couple weeks after we get back"). If the problem recurs during today's sessions, please send email to them at multicast@xxxxxxxxxxxxxxxxx so they'll see that I'm not a lone crank.
Melinda