You play it in CLIENT and not SERVER. The server has to be updated for it and that's not happening as soon as a snapshot come out.
You'll need a separate minecraft.jar file in the root minecraft folder, and have to use a program that renames them for you or you'll have to do it manually when wanting the current one for the servers.. If you never edited your stuff then you may want to check out some YouTube videos on how to do it. they are out there.
Since this is apparently hard to Google, I'll make a nice, easy walkthrough for you, and whomever else needs it. Moved to this thread.