Right now subscriptions can not be purchased through the iOS app (via iTunes). In the past during Spotify's introduction offer, users would accidentally subscribe through the app instead of the website causing a lot of confusion. This decision was made to make the offer go more smoothly. Sorry for the inconvenience.
You can still subscribe to premium via http://spotify.com/premium
The option to subscribe from inside the iOS app will be coming back at a later date.
Tweet from Spotify support on Twitter:
"During our Intro Offer, in-app purchases are not available. Get started with Premium at http://spoti.fi/1qSoQB4"