Cropmarks are only displayed in movie mode by default (liveview or playback). Also make sure you have them enabled in the menu.
This is mentioned in the user guide:
http://magiclantern.wikia.com/wiki/Unified/UserGuide#PLAY_mode_shortcuts
This is mentioned in the user guide:
http://magiclantern.wikia.com/wiki/Unified/UserGuide#PLAY_mode_shortcuts


