I recieved the game as a gift from a friend and I am totally stuck.I have tried the net for answers with no luck. What I need is a walkthrough if anyone has one for this version.

The box the game came is Ghost Recon Advanced Warfighter, Limited Special Edition. It's for xbox.
Recruit:
When the game starts I am on top of a overpass under construction where I walk around a corner and down some stairs. I proceed forward and kill enemies down this street and then end up going to the area where the helecopter is and then the copter takes off and I am locked behind this gate that does not open?
If anyone can help I would appreciate it, I've tried walktroughs, forums and everything.Just about everything refers to a different version.