To use the following code download the appropriate help and class files, and place them in your Extensions directory. To build the plugin itself you will need Xcode or another compiler. See the notes on this swiki or in the SuperCollider main help.
On OS X I modified the xSC3plugins project in Xcode by adding a new target (I just duplicated an existing target, right click gives a menu of options for duplicate rename etc on existing targets, and also lets you add existing files for the cpp source). You need to edit the target settings by double clicking on the new target. Make the product name ACMCdemo.scx or similar. Use GCC3.3 for backwards compatibility with OS 10.3. You just need the compile source to be the ACMCdemo.cpp file below. On my system I add the following lines to the build Run Script option page:
cp build/ACMCdemo.scx build/plugins cp build/ACMCdemo.scx /Volumes/data/sc3code/scwork/plugins/
This then makes a local copy in my SuperCollider build and also copies the new plug-in to a special plugins directory I maintain. You might have this anywhere- I have a symbolic link from the Extensions folder so these are automatically included in any running copy of SC. Strictly speaking, in this case the first line is unnecessary. I use Xcode 2.0, there might be slight changes with later Xcode versions.
The plug-in here is just a simple random distortion effect. In the workshop I just showed the effect of changing a number in the C file and recompiling, just to show the sort of thing involved in editing plug-ins.
C source code file