If you can upload programs but still get that error message, it's a problem with RXTX (which is the Java library Arduino uses for serial communication). It may have something to do with the fact that the macosx_setup.command didn't run properly the first time. If you open a Terminal and type "groups", does the resulting output include "uucp"? You might need to delete the contents of "/var/lock" (which you can get to from the Finder: "Go > Go To Folder...").