How can I go about deauthorizing iTunes from my old computers?

I've seen the instructions on deauthorizing old computers by going into the account settings and clicking deauthorize all, however, I can't find that button anywhere. Has the interface changed since those instructions were published? How can I go about deauthorizing my old computers? I need to be able to authorize iTunes for my new Mac and already have 5 computers authorized.

[Re-Titled By Moderator]