On 2009-03-13 I Google'd for "VLC Mac", downloaded vlc-0.9.8a.dmg onto galway, and then copied VLC.app to /Applications.
To view a multicast stream, start VLC and select File->Open Network.... In the resulting window, fill in the MRL or click UDP/RTP Multicast and fill in the Address and Port fields. If you fill in an address and port that don't correspond to a valid multicast stream, nothing will happen. A valid stream is the "TA 3" TV channel from Bratislava, Slovakia, with the MRL rtp://@233.10.47.11:1234. Note: I couldn't figure out how to tell Mac OS X that it should use VLC to play "RTP" MRLs, but I bet it's possible. I thought "Default Apps" would let me do it, and it almost does, but "rtp" isn't listed in the URL type list. Another way is to go to http://www.multicasttech.com/ and with the cursor over an SDP link, right-click and do Download linked file or Save As... to save it to the Desktop, then drag the icon on top of VLC and release.