I have used this keyboard shortcut several times in my tutorials to load the image luminosity as a selection and a lot of people have been unable to make it work on a Mac system. This is because the Mac OS has this particular keyboard command assigned to another system function. To fix this you can go into the Mac OS System Preferences under the Apple menu. Go to Keyboard and Mouse and click the last tab called Keyboard Shortcuts. Scroll down and twirl down Keyboard Navigation, then scroll down to ‘Move focus to the window drawer’. Turn this feature off by unchecking the box next to it. Now this keystroke will work inside Photoshop.