@Stick Pin You don't have to be sitting in the seat to do this. You actually should do this before spawning a Sundy because the AMS attachment won't be on the vehicle until you equip the AMS attachment to your Sundy. And it won't just appear on an existing Sundy if you add the attachment after you've already spawned it.
You press ESC to bring up the main menu. Then you select the button that shows a tank for the icon on it. If you hover over that icon it should say vehicles. Then you will see the list of vehicles. Select the Sunderer. Now select Loadout on the right hand side. *NOTE: I named my loadout S-AMS. I think it's called Default or Loadout 1 normally.
On the Sunderer loadout menu select the Utility button
Then the menu will open up the attachment slots for the vehicle. Add the AMS attachment to it by clicking on it.
The preview picture of the Sundy will show an equipment term on the sides of it now. The utilty slot also shows the AMS attachment as well. You will be able to press B to deploy the AMS now.
EDIT: Added pics
EDIT: *Game Update* AMS terminals now work on the sides. Terminals have been removed from the back.