In either the pre-build or post-build command line properties of your project. (project->properties->build tab)
C:\pathtoyourflexinstallation\bin\mxmlc.exe $(ProjectDir)\src\skin.css -output $(ProjectDir)\bin\skin.swf -library-path+=src\flex_skins.swc
You should be able to use $(CompilerPath) but that doesn’t work at the moment.
More details and future fix
