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.
You can use KeyRemap4MacBook to remap most keys on your keyboard. Particularly I used the option which maps the Eject button to F13 and Fn+Eject to Eject.
You can find it here!
consider using a different key than Eject; since it sends KeyDown and KeyUp events at the same time the system will not notice you trying to hold it. Also, it does not send any event until a small delay time has passed, during which you must keep it pressed. This can be very annoying if you are a fast-typing maniac and wish to be able to hit it very quickly. However, delay time is smaller than 0.1 seconds on my keyboard, so you will hardly find it an issue
Another useful tip
It is also useful if you liked old MacBook Pro keyboards which made a distinction between Return and Enter keys (very useful with Wolfram Mathematica for example) to remap the right option key to Enter. It has loads of other options, you will surely find it suitable for your needs.