The iPad Mini does not charge using a laptop
My brand new iPad Mini does not charge using my macbook pro, but it does when is connected to de ac power wall adapter !!!
iPad, iOS 6.0.1, iPad Mini
That is the way is is supposed to work. Your MBP does not supply enough power to charge the battery efficiently. It may be charging, but at a trickle rate and it will show as not charging. That is why a power adapter is included with the iPad. Use it.
Read more about charging the battery here.