iPod Touch 5: Start new messages field doesn't show up
Is iMessage activated on your iPhone?
Is your iPhone and iPod signed in with the same Apple ID and connected to the Internet?
Without activating iMessage on your phone with the same Apple ID you can't use the telephone number on your iPod.

If you can't use the phone number for iMessage on your iPhone, then try with turning iMessage off from the settings and turn it on again a few seconds later. Try doing that on the iPod as well.