Several of our forum members had difficulty activating the feature, as the required activation code would never appear on their Macs or iPads.
MacRumors reader Michael wrote in with this solution which requires an email address to be activated in iMessage:
...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.MacRumors was able to verify this scenario and found that an email address does indeed have to be active on your iPhone for the activation message to properly be sent. In our testing, SMS messages were still received after the email address was removed from iMessage. The setting to add your email address to iMessage is under Settings -> Messages -> Send & Receive. Note, you may have to wait a few moments after adding an email address before the activation code will send.