Without an example, it is difficult to say what the problem is. So I will give just a general description. The Zoo Admin Download sites are set up so that guests can download things without being members. The download sites do not allow registering. There are 2 Zoo Admin download sites: 1 for ZT1 and 1 for ZT2. The ZT2 download site is here:
http://www.zooadmin.net/ZT2/index.php?action=forumZoo Admin had problems in the past. The last problem was that the computer server it was on became corrupted. So the web host company put Zoo Admin onto a different computer server. But it is Zoo Admin staff that has to put the things back onto the new server. That is happening now, as the Zoo Admin staff find time. So there are many broken links at the download site, which will give "404 Errors" until they are fixed. There are also many things that are hidden until they are fixed.
You might have gone to the Zoo Tycoon Community Download Directory (
https://www.zooadmin.org/DD/index.php?action=forum). Although that site is owned by the same person (Yellowrose) whom owns Zoo Admin, that site is not related. That site has lists of downloads that are at many Zoo Tycoon sites, including Zoo Admin. If you click a link there, it goes to the site that has the download. Many of the Zoo Admin links there were made when Zoo Admin was on the previous computer server. So if you click a link there, it might try to go to a hidden area at Zoo Admin's download sites, which would then give you a message that you need to log in. That means those areas are not currently available. In some cases, the Download Directory site was updated to say "ZAOffline" in the lists, but probably not everywhere.