Hi Matt... I just ordered Sonar x4 pro last week. (sadly it isn't here yet)
The way I understand it, once you have Sonar installed you have to go to the Cakewalk site to activate the mp3 encoder. You have to give them your product license number in order to do so. The good news is that Cakewalk doesn't charge you $16.00 anymore. It's now free.

Bobby