OK, I've had a look. After you've taken the Imperials key (for later use) step onto the elevator and push the control panel on the wall using 'use' in my system it's the E key. Go up a level, go outside onto the balcony and turn right into the lift shaft. step onto lift on right and again push the control to take you up into the control room.
There you will find the 'outer defences' controls. Push again to activate the gun outside then retrace your steps back outside.
Jen will be waiting at the gun.
Use push to access the gun and blast through the main outer doors of the building. After blasting press jump to release control of the gun - join Jen on the lift (checkpoint) which will automatically take you down to the lower levels of the installation where you will find lots of Imperials to shoot at and lots of nice puzzles to solve!