Do you have JAVA_HOME set to a JDK?
Mac OS X has a weird JDK layout. They basically rename everything, especially tools.jar is named classes.jar and doesn't reside under JAVA_HOME/lib.
If I remember correctly it's in JAVA_HOME/LIbraries. So all you need to do is to symlink JAVA_HOME/Libraries/classes.jar to JAVA_HOME/lib/tools.jar.