Hi there DjPolar,
You might need to run a cable out of your mixer and into another computer / another soundcard to record. Run the master out of your mixer into the soundcard for broadcasting. In Shoutcast, select the line in of that soundcard as your source.
However this is from the Winamp Shoutcast page:
"Broadcasters need to use Winamp and the SHOUTcast Source Plug-in for Winamp. There is no current solution for broadcasting from Unix or Macintosh"
So i would assume that they do not support Shoutcast on Mac.
You would be better off contacting them on their own support page >
forums.winamp.com
Cheers!