Power up the unit and connect a terminal to the RS232 (craft) port. Verify that you can get a prompt. Call 1-888-4-ADTRAN. Enter "CHALLENGE" as the username. Provide the friendly folks at Adtran the characters that appear on the screen. They'll tell you what to enter as a response. Change the password once you're in.
I suggest you try the factory default ADMIN and PASSWORD. If the unit's previous config wasn't saved via "write memory" command, power cycling the unit will restore factory default options.