Do you have Airport 4.2? According to the link I gave, this version is required in order to enable NAT-PM. Check software update to ensure you have already installed the aireport update.

If you can't get anywhere with this, then manual port forwarding it the way to go. If you're concerned that port 6346 might be blocked or shaped by your ISP (internet service provider), then try a port between 49152 - 65535. Likewire you'd need to put this number into LW's manual port forward & also the Listening port. In fact, before you do manual port forwarding, try changing the listening port to one in the range suggested; eg: 64049. And see if that makes a difference.

BTW who is your ISP?

Your version of LimeWire, did you mean version 4.0 or 4.10 or another. Please let us know.

BTW I'm guessing your admin password for the airport will be the same as your Mac. If not, ask whoever else has admin abilities for the computer. Perhaps you haven't set one up for the airport yet. Did you check the manual?