I recently have found need to have my function keys to perform their alternate functions full time. As in function as "F" keys without pressing "Fn". I looked up how to switch everything over in Apple Help and this is what it said:
Hi there atatzak!
I believe the article you are looking at may be for newer versions of the operating system. I have an article for you that outlines how to do this on older versions of the operating system, like the one that you are running. That article can be found right here:
Thanks for using the Apple Support Communities!