This is buried in the docs as a comment on this page: https://developer.ibm.com/watson/docs/developing-watson-apis...
Edit:
No real support for 'playing around' with the API. Bummer.
Edit2:
Just went through the application process linked above. Be prepared to give info about yourself and your company and an explanation of why you want access to the Watson API, as well as what type of information you'll be working with. I stated 'just want to play around with the API'. We'll see how they react to that.