Nicecast F'n rocks!!!
I love that app!
the thing that you need to do, is open a port in your firewall to let it "get out"
I'm away from my mac right now, but there is a section (maybe in the prefs) where it tells you what port you need to add.
when you find the port number, go to system prefs>sharing>firewall>add and add a port, name it "nicecast" and type in the port number.
that should do it...then just start playing iTunes, and click "broadcast" or something like that in Nicecast....
Everything but the port thing is pretty easy to figure out.
I love it, I'll start a broadcast at home, and then click on the radio link it gives when I'm at work and listen to all my iTunes music at work!!
Plus you can add AU plug in's to the radio show, so I'll add crazy plugs, like a vintage warmer, or reverb just because I can.....
great app
good luck!