I am currently running OSX 10.9 and have been trying to use mechjeb in sandbox mode. I currently have Core anvil, chatter and RLA electric engines add ons installed and have tried placing mech jeb in the files gamedata>squad>parts>utility and in the parts folder seen on the page next to game data, saves, screenshots etc. The parts appear in the VAB and can be attached to rockets but unfortunatly I have not been able to access the MJ controls on the launch pad. Hop this information is useful. Any help would be appreciated.