Ecco una cosa da provare, da MacRumors :
...you need to have your email address turned on for iMessage on your iPhone in order to enable Text Message Forwarding. If you don't, the numeric access code will not appear on your iPad or Mac during the setup process.
As soon as you enable your email address for iMessage (you only need to do this on your iPhone) the numeric access codes appear as expected. Once you have text message forwarding setup you can disable your email address again in iMessage as it seems to only be necessary for the numeric access code setup step, not the actual text message forwarding itself.