The DVB-S2 dissector has not yet been included in Wireshark releases(not even the development releases), however it should be available in a future release since I can see the dissector code in the trunk. See this question for more.
I'm able to dissect DVB-B2 packets using my local Wireshark build. So if you need this feature right away, I guess the best option for now would be downloading the source and using a local Wireshark build for your purposes.