To be able to post to Twitter from Google Wave, you must use a Google Wave bot called Tweety or Twitbot (others call it Twave although the bot is officially called Tweety). To add this Twitter bot, go to your contacts in the bottom left corner of Google Wave, click the "+" sign and add this:

tweety-wave@appspot.com

To be able to twit from Google Wave, create a new Wave, click the "+" sign on top of it and add Tweety the Twitbot participant to your wave: