"Career Officer" is a title, just like Captain or Special Envoy. You can find it on your captain's paper doll towards the bottom of the window. It should show up as a drop-down menu.
You have to go to the C-Store first, click on the "Special" tab (or something like that i forgot the name) and unlock the Career Officer title. Then you can change your rank from the character screen (default key U).
Comments
Oh, yeah. Forgot you had to claim it first. D'oh!