Hmmm... perhaps google something like "move itunes from c to d" and see what you get.

I don't mean to be nasty, but dude if you've already had a google of it then fair enough
but do let us know that you have. Without googling I would say you're barking up the wrong
tree. Imagine asking "how can I tune my WRX" on an iTunes forum. Sheesh.

http://www.JustFuckingGoogleIt.com/