I had the same problem at first. I got it running by:
Make sure you are running the program from the /usr/local/bin/napshare directory. Next, manually enter the router.limewire.com:6346 host. Give it a minute or 2 to download all of the other hosts. If it still doesn't let you add a host, try running the program by su'ing to root first.
Hope this works for you also!