In order to have MarkLogic start up on user login on my Mac, I created the folder ~/Library/LaunchAgents and put this .plist file into that folder. After logging out and re-logging in MarkLogic had been started successfully.
Update - May 23, 2011
I've updated the .plist file to restart instead of start. When MarkLogic starts it doesn't check if it's already started so it will keep the other MarkLogic instances running.