I was looking through the System Preferences for Mission Control in OS X Lion (10.7). One of the options had F13 in it but I couldn't find it anywhere.
Where is it?
MacBook's don't have the extended keyboard which contains the additional 7 function keys (F13-F19). They're only on Apple's full size ( with numeric keys) keyboard as presented on this photo:
Apple laptops don't have function keys higher than F12.
Certain Apple keyboards do, however.
When using an external IBM/PC keyboard, PrtScn/SysRq, ScrLk and Pause Break should map to F13-F15.