You need to use a FOB to access the navigation computer

You need to use a FOB to access the navigation computer is a quest in Fallout 2.

Walkthrough
The Tanker FOB located at the Enclave base of Navarro is in the commander's office.

To get into the Commanders office you can Attack on sight, sneak into, or lie to the guard and simply walk in posing as the cleaning crew.

Afterward head to the tanker in San Francisco and on the lower level of the tanker use it to go to the Enclave Oil Rig. Be warned once you do use it, you can not come back to the mainland until the game has been completed.