There is a very important trick to play before installing Oracle JDeveloper on a Mac. Basically you need to fool the JDEV installer to believe the java on your Mac is like those on Unix/Linux box.
cd /System/Library/Frameworks/JavaVM.framework/Versions/1.6.0/Home/
sudo mkdir -p jre/lib
cd jre/lib
sudo ln -s ../../../Classes/classes.jar rt.jar
This step was actually on Installation Guide for Oracle JDeveloper