You do have this option NAT-PMP setting instructions for Airport

Else manual port forwarding might be safer. Airport port forward instructions but if you don't have the base station option then see alternative instructions; 1. Set up Port Forwarding on your Airport (don't choose dmz! Choose port 6346), or this link 2. And choose one such as one of the bittorrent set of instructions but use port 6346, or this one Another instruction option without base station

Is the Airport the only connection device you have?