iMessage from a phone number
In iMessage I only see options to send and reply using email addresses. How do I add a phone number?
iPad
You can only add a phone number if it is an iPhone number. It doesn't work with other cell phone or android phone numbers.
IF you have an iPhone, you will probably have to sign out of Messages on your phone and on the iPad and set it up all over again, starting with the phone. That should make the phone number accessible in the settings on the iPad. Read this and pay attention to the information at the end of the article.
iOS and OS X: Link your phone number and Apple ID for use with FaceTime and iMessage