How can I use the function keys on my MacBook to control itunes?

You might want to try Synergy.
http://wincent.com/a/products/synergy-classic/

It is 5€ but you can try it for free and see if it does what you want.

I use a great program called Spark to assign the f-keys to my own functions. It has built-in support for iTunes, or Synergy is good too. Also, because Apple has predefined functions for the function keys on the macbooks (keyboard brightness, volume, etc) you either have to use the unavailable ones or press the “fn” key if you program functions to those other keys. Or, if you want your function settings to have priority you can switch the operation of the function keys in the keyboard system pref pane.

coversutra makes all that easy

you should check out quicksilver: link

i used it to set up my itunes shortcuts, and i use it to quickly find and launch apps/docs.

and it’s free.