I recently did an uninstall/reinstall, so I could try this. I added the String: SpecialFunctions with value 1
in the registry as JAYP says, and F2 works fine.
You have to be in the Mission Builder with a mission open, not in the CAMPAIGN Builder...just in case that's where you tried it.
Otherwise, unless you made a typo, or put it in the wrong spot, I can't imagine why it isn't working.