Why can't I message on my Ipad?
I don't believe any carrier offers straight SMS from an iPad so you will either need an app that contacts an SMS sending service from the App Store or to ensure that your recipient signs up for iMessage.

Apple provides iMessage for free and it replaces SMS as opposed to interfacing with the SMS.

So, you could only send iMessages to Computers that run iMessage and iOS devices that run iMessage - but not actually send a SMS to cell phones.


http://www.apple.com/ios/messages/
http://support.apple.com/kb/TS2755


If iMessage seems to be something you want to try - sign up for free according to the instructions on the first link. The second link explains the difference between SMS, MMS and iMessage services and when they can commingle on iPhone. Again, iPads only support iMessage out of the box (i.e. without adding a third party app).