How do you register a phone number that isn't registered with iMessage?

Do you mean that you are trying to message someone and are getting a message that they are not registered with iMessage? If so, that other person needs to sign into iMessage. This assumes, of course that they have an iPhone, iPad, iPod Touch or Mac computer.

How do they sign in? I'm using my iPad Air 2 on iOS 9.2.1 (up to date) and my friend has an iPhone, his iMessage is turned on and the "send & receive" tab is linked to his phone number. But when I try to message him off of my iPad it says this number is not registered with iMessage

im all logged into iMessage and can text other iPhone users no problem