Using OS X Mavericks? Click here

1. Open your Mac's System Preferences
   Click on the Apple Logo in the top left then click "System Preferences..."
   
   
   
   
2. Select the Universal Access option.
   Under the System options click the "Universal Access" icon and it will open a new window.
   
   
   
   
3. Check "Enable access for assistive devices"
   At the bottom of the Universal Access window you will see a check box labled Enable Access for Assistive Devices. Make sure that option is selected.
   
   
   
   
4. Reboot your computer.