On more recent versions of Mac OS X there is a setting which prevents ScreenRuler from spanning over multiple monitors. To fix this, you will need to switch a setting off in System Preferences:

  1. Click on the Apple logo in the top left of your screen
  2. Select System Preferences
  3. Select Mission Control
  4. Uncheck Displays have separate Spaces
  5. Log off or restart your computer
  6. Launch ScreenRuler – the ruler window will now span over all your displays