Dragonborn option is only given when you get to that point I the MQ, you either bypassed that point somehow or don't realize your already a member.
Don't tell me its not possible to come back to it.. i feel i may cry, im NOT learning magic, im NOT starting again, and it pisses me off how i can only get every dragonshout and priest mask by doing EVERY guild! Even the Forbidden Legend quest requires me to finish the college quests! This is [censored] stupid Bethesda, sort this out! This is the only problem ive had with this game, everything else i can work around, or look past, graphical bugs are not an issue to me, but when [censored] poorly thought out gameplay stops me from enjoying the game i get mightily pissed off!