Firstly, make sure it's the latest one, the old one didn't allow it. Secondly, make sure you're in command of the helicopter. Select the "Hold Station" option and the pilot will put the helicopter into a hover. You'll get a message when it's safe to release your harness, and when you do you'll be able to wander around the back of the chopper. If you want the heli to move the helicopter, select fasten harness, then select the "cancel hold station" command, and the pilot will start flying as normal.