Hi, and welcome back! As per the "latest news" items and Announcements forum, there have been some very recent (within the last couple of weeks) changes to the site, and while most things are working well there are still some items on the to-be-dealt-with list.
This thread in the Site Questions forum ( http://www.bookcrossing.com/---/525635
) goes into the prenum-label-download question, and includes a link to the "labels" page. However, while you can allocate prenumbered BCIDs via those links, it's possible that printing them from that page is currently a "wings" feature.
That page also has a link to the blank BC label form, so you could allocate sets of prenums and then enter them by hand in that form - or use one of the member-designed label templates. If you only register a few books at a time, there may be no advantage to using prenums under those circumstances, as individually registering the books might be quicker, but if you want sets of prenumbered labels available for quick use, that could be a workaround.