I’m using a sleek Apple / Mac keyboard on Windows OS and I had troubles getting a screenshot. Here’s how I got it done:

1. Download SharpKeys.
2. Install and run program then add a key mapping like this:
   
3. Press Write to Registry.
4. Logout/Login or reboot.
5. DONE!

Note: in my example I associated F13 with Print Screen but you can choose another key.