A better and simpler way to do this:
Locate the Compile script in the SystemModeler bundle, in a standard installation it should be located here /Applications/SystemModeler.app/Contents/MacOS/Compile.
To get there, right-click (or Ctrl+left click) on the SystemModeler bundle and select "Show Package Contents". Then you can browse into Contents/MacOS. Open the Compile file in a text editor and substitute the contents with this:
export export MACOSX_DEPLOYMENT_TARGET=10.8
make $2 -f $1.makefile > $1.log 2>&1
WARNING: Make sure that the new file does NOT use windows line endings, because then nothing will work.