If it detects a firewall, then the problem will most likely be your airport.
AFAIK Airport does not support UPnP per se. But they've developed something similar which should work with UPnP settings. See bottom of this following page under the section called
2) NAT-PMP http://www.rogueamoeba.com/utm/posts...05-07-25-10-10
The article is re: Nicecast using UPnP, but should also be applicable to LW.