I just had the same issue and this is how I got "Text Message Forwarding" to appear in my settings.
I first updated the iOS on my iPad since I was using an earlier version than 8. You have to be at least using iOS 8 for "TMF" to work.
I then followed the instructions on my iPad to get it set back up. The problem I had there was it kept saying it could not activate my Apple ID for iMessage and FaceTime. I looked in the forum and found a simple solution for that. I turned my iPad off for a few minutes then turned it back on. I was able to sign in to my iCloud account then with it actually activating. After that, I had to also sign in to iMessages.
Once I had done all that, the "TMF" option appeared in the settings on my iPhone under Messages.
I am not sure, but I think it does not appear for some people because they have not set their iPad up yet. Once, "TMF" shows up on your iPhone, it will list the other devices set up for iMessages that are on the same iCloud account as your phone. Therefore, it makes sense to me that the "TMF" option would not be there if some other device had not already been set up for iMessages.
I already had my iPad set up for iMessages, but decided to use the Continuity option so that I can receive text messages other than just iMessages.
Hope this helps!