iMessage signin
What do I need to use iMessage ?
iPad Air, iOS 8.1
You need to turn Messages on in the Settings. Settings>Messages>On. You have to be signed into your Apple ID as well. The Messages app works over WiFi and you can only text with other iOS users who have already set up messages on their devices. If you have an iPhone, you can link your phone number to your iPad and use text message forwarding from the phone to the iPad in order to send standard SMS messages to non iOS devices.
Send messages with your iPhone, iPad, or iPod touch - Apple Support
iOS and OS X: Link your phone number and Apple ID for use with FaceTime and iMessage - Apple Support