Ok, I've tried under the skins section, but when I go to save it they say you have to be a Premium user; then on the Look & Feel page under HTML I put some things and the next time I go back it's gone. Can someone tell me the exact place I'm supposed to put the HTML/CSS for my Xanga site? Thanks.

Usually, it's:
HTML/CSS -- Custom Header.
Javascript -- Website Stats.

This does not apply to all cases.