Update: If you are using Kettle 5.01 on OS X Mavericks, there is a different issue that causes problems. Go here for more info.
A quick note for anyone who may be trying to use the Pentaho ETL tool (Kettle) on a Mac with OS X Mountain Lion, and running into problems getting it started. There is a permissions issue that needs to be fixed within the application before it will run. Its not difficult to fix, but it does require you to use the Terminal application.