I couldn't get it to work either until today when I downloaded a free program called Port Map which makes it so easy to map UDP 5198 and 5199, and now Echomac works just fine.
Hope it works for you.
Just download Port map and open it.
Click on the + mark, check off the UDP box and type in port 5198.
Repeat the process typing in port 5199.

I had previously tried using Airport utility to open up Airport's ports and just didn't have any luck.
Best of luck.
Mike/KA1RTW
mgrace@maine.rr.com

I couldn't get Echomac to work until I downloaded a free program today called Port Map.  It allowed me to very easily map UDP ports 5198 and 5199 and now Echomac works fine!
Hope it helps you.
Mike/KA1RTW
mgrace@maine.rr.com

Mike:  I had a similar problem here - could see the list but not connect to anyone.  Then today I downloaded a program called Port Map for free for OS X on the internet (do a google search).  By allowing UDP ports 5198 and 5199 with this free program, echomac works fine now!
Hope you have a similar success.
Mike/KA1RTW
mgrace@maine.rr.com