If TurboTax thinks you were finished and intended to file by printing and mailing, but you really haven't filed anything, you can open the return back up to change the filing method and to proceed with efiling.
At the Tax Home or in the section "Your Tax Returns & Documents", look for a small box with a link that says "Add a State." (You don't really add one; that's just to open the return back up.)
Once the return is open, you can continue working on it.
Once you've finished it, go through the REVIEW section.
If the Review is OK, then go to the FILE section, change the filing method to efile, choose a refund method (if applicable), make any payment (if applicable), then continue on answering screens until you get to the last screen which will have a big orange button "Transmit My Returns Now" and click on that.
