You can find the component you need at
http://downloads.xiph.org/releases/xiph-qt/xiph-qt-0.1.8.dmg
I've given you the direct download link so if you can't interact with html
when you get there don't worry; just now when i did it I found html content
that wouldn't interact and as soon as I closed that window I saw the
downloads window but the file had already downloaded. Also I'm typing this
in windows and the url didn't appear to stay on one line so if you have any
trouble just copy and paste the url from your email into safari.
You must copy the component which you will find inside the .dmg file and
paste it to either Library/Components in your home directory or/
Library/components on your sytem, inside Macintosh HD. On my leopard system
there was no Components directory in my home directory but there was one in
/Library so I copied it there; you can of course create a Components
directory in either place if it isn't there. There's a .rtf file inside the
.dmg that explains about installation also.
Btw this is a frequently asked question and checking the list archives or
searching for "itunes ogg" in google also would allow you to find this
information.
Hth.