Q: how do i enter an itunes card on this ipad?

You can do it in either in the iTunes store or App Store apps. In the iTunes app scroll to the bottom of the Music tab (or Films, TV Programmes, Podcasts or Audiobooks tabs) and there should be a Redeem button (similarly in the App Store app on the Featured, Top Charts and Categories tabs).

How to redeem a gift card : http://support.apple.com/kb/HT1574