In the Personal Info/My Info section, you should indicate that you served in the US Armed Forces in 2021 (look for it).
The next screen should ask you what your military status was at the end of 2021, as well as were you in the military all year.
The screen after that asks if you were enlisted or officer. If you check "enlisted", a pulldown menu should prompt you for the rating.
The interviews between Online and CD/desktop are slightly different, but not so different that you can't figure it out if you are flexible.