windows 7 - How to add item to right-click menu when NOT selecting a folder or file?

How do I add a menu item to my context menu to open up powershell (or any other arbitary application) when I right-click inside Windows Explorer?

I found tons of links on how to add menu items when doing this: (selecting a folder; example link)

...but couldn't find any instructions on how to add menu items when doing this: (not selecting a folder or file)

I did find this and tried adding powershell to HKEY_CLASSES_ROOT\Directory\Background\shell\, (loosely based on the instructions I found and linked above) but it didn't work for me.

I'm running Windows 7, by the way. Also, I'm currently interested in adding only PowerShell, but I'd prefer a general solution I can apply to add any program to the context menu.