By "pre-register" I'm assuming you mean online check-in?
If so, there's info on that
HERE
When faxing your site/loop preference, keep in mind that it's just a request, not a guarantee, and while they usually do their best to accommodate, you're checking in at probably the busiest time of the whole year and it may be hard for them to meet exact site/loop requests. Probably better to tell them specifically what it is you need/want in a site - i.e., near comfort station or bus stop, room for tent, etc..
Also be sure that what you request actually meets the category of site you've reserved. For example, if you've reserved a Preferred site there are only 3 loops - 100, 200, 300.
Oh - and you don't want to fax your request until about 4-5 days before you arrive.
Good luck and have a great trip!!
