RTMPT just not showing up…

I'm trying to decode RTMP (real-time messaging protocol) in Wireshark. I just installed Wireshark v1.6.2 on Windows Vista 64 bit. The RTMP packets show up in the trace but are not decoded. The tcp port number if correctly displayed as macromedia-fcs, but the packets are displayed as TCP frames with undecoded data payloads. When I right click on a frame and select "Decode As...", RTMPT shows up no where in the interface. Lots of protocols, but no RTMPT. Any idea what's happening? Do I have to install a separate plugin or some such thing?

Thanks so much! And by the way, this tool is just amazingly useful.