I also installed it on my iPhone but I think you have to log a certain number of attempts (3? 5? I don't know) before it gives you the upgrade option.
Assuming the ipad version is identical to the iphone version. There is a button in the top left corner of the screen to access your profile. This menu gives you the option to either sign out or upgrade to premium. No information regarding what the upgrade does is provided. Being as the prices are identical I assumed this would give me the fully featured app. Reading the forums, it would seem I'm far from alone. I've since paid for the full app however, I still feel that this matter could be handled far better by GS.