We are working on including direct Shoutcast/Icecast support in Mixxx. It currently has some problems that are serious enough to prevent us from turning it on by default. However, the more adventurous among you are free to build Mixxx v1.7.x or later with the shoutcast=1 flag to give it a try.
In the meantime, please use the following method that has worked well for one of our users for years.
Hello, all! It's thread here with a quick description of how I have Mixxx stream out to dnbradio.com's shoutcast servers every Tuesday.
I run mixxx under the Ubuntu-based linux distribution, Crunchbang Linux. Using Jack, I feed Mixxx's main L/R outputs to the simple, console-based edcast (http://www.oddsock.org/tools/edcast_jack/), which is then able to encode the audio to mp3 in realtime and feed it up to the remote server.
I fire up Jack using the very simple frontend, qjackctl. Then I start Mixxx and set it to output its audio via Jack. After some straightforward editing of the edcast configuration file, I launch it, specifying the configuration file and the jack sockets to stream from, with something like this:
~/apps/edcast$ bin/edcast -c etc/edcast.conf PortAudio:out_0 PortAudio:out_1
And if all goes well, edcast will start telling you how many kilobits were sent up every second, until it is killed with ctrl+c.