In order to get launch4j to work on a macbook air running OSX Version 10.7.5, I had to do the following:
1. tar xf /data/launch4j-3.1.0-beta1-macosx-x86-10.8.tgz
2. sudo port install i386-mingw*
3. cp /opt/local/bin/i386-mingw32-windres launch4j/bin/windres
4. cp /opt/local/bin/i386-mingw32-ld launch4j/bin/ld
None of the available prebuilt binaries worked out-of-the-box, but the above steps did the trick.