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 understand better, take a look at this image:

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:

Make sure you allow pop-ups from Google Wave, as a new window should now pop-up to allow you to authenticate to Twitter. After successfully logging in, this is how the new Wave will look:

Now you can start posting to Twitter from Google Wave.