Usually, a device that is not attached to an iFly account will remind you of that situation when iFly launches and ask if you want to log in and/or purchase a subscription. Otherwise, tap the Menu button, and the top option in the menu should be an option to Register or Log in:
If you don't see such an option, then the device is probably already registered to your account. You can confirm this by Menu > About > My Account. That page will either show you information about your account and when your current subscription will expire, or else it will show you options for purchasing a new subscription.