1. Pay your international departure taxes at a ticket counter or customer service station before going to the gate. This is also where you can find out if a seat is even available; unless you know an ATN crewmember who can look up loads on the spot from their smartphone or computer.
2. There must be a seat in the cabin available for you. Only AirTran pilots can ride on the actual cockpit jumpseat on international flights. TSA thing that applies to all airlines. Even though this rule was rescinded some time ago, most airlines haven't modified their procedures yet. I think it has something to do with Gen Dec Homeland Security database verification of inbound OAL crewmembers.
3. If any gate agent gives you a hassle, just explain that you are a jumpseater travelling in a cabin seat, not the actual cockpit jumpseat. If that doesn't work, ask to speak to the Captain of the flight. He/She will fix it provided that 1 & 2 above have been taken care of.