lock screen shortcut with windows keyboard
I'm running Yosemite on a late 2009 iMac and recently switched to a wireless (HP) keyboard and mouse. I am using a package called "Karabiner" to enable brightness, volume, and media control shortcuts with the F1-F12 keys. Now I am looking for a way to make a shortcut to lock my screen like I did with ctrl+shift+eject on the old keyboard. When I am in system preferences -> keyboard -> shortcuts -> app shortcuts, I hit the "+" button, set Application to "All Applications", set Menu Title to "Lock Screen", and when I type in the Keyboard Shortcut box nothing appears. The only keys that show up here are those from the numpad. I really need a way do lock the screen quickly from the keyboard.
iMac, OS X Yosemite (10.10.1)