Telika: I had manually typed (or copy/pasted) the name of all the contestants in a separate .txt file, and copied/pasted the list on random.org.
bevinator: This is basically what I did, because if you just do it by post numbers then you get messed up by multiple posts from the same person or by posts that aren't entries. I just put them all in a list, and then used the random integer function to pick one.
I'm sure it's possible to script doing that rather than by hand. Unfortunately, the site is optimized for display, not for running this sort of contest.
I took a look at the page source and it looks like one could filter for(with the rest of the tag, obviously):
div class="small_user_name" hedwards /div
div class="small_icon" /div
div class="small_text" Comment buried. span id="unhide_7" class="unhide" Unhide /span /div
and remove the newlines to create a list of posts. Then one could eliminate any usernames that show up more than once. Followed by appending a line number for use in the contest. And one could manually verify that the person intended to enter before announcing it.
All that is more or less trivial to do in *NIX, not sure how you'd do that in Windows though.
EDIT: Or just have it shuffle that list.