How do I set up iMessage on my iPad. Unable to receive/send messages with my phone number

Hi! I went through the tutorial on setting up my text messaging on my iPad, but it won't allow me to add my phone number; it only defaults to my email address. I feel as if i have typed in my apple account password 50 hundred times, but to no avail. I even tried to manage my apple account, thinking i needed to add my phone number there but my phone number is already on my account and will not register under "Settings", "Messages" Please help!