Welcome to Zoo Admin and welcome back to Zoo Tycooning.
The Zoo Admin ZT1 download site has all of the official downloads, including the Atlantean (Atlantis) Theme Pack. But we put the download sites offline near the end of the month to save bandwidth for this main site. They will be back online in a day or 2.
As for finding specific downloads, there is a site called the Zoo Tycoon Community Download Directory (ZTCDD or DD for short). The site is still being worked on and is not up-to-date yet, but it still can come in handy to find specific downloads. For example, here is the listing for the Atlantean (Atlantis) Theme Pack:
http://www.ztcdd.org/DD/index.php?topic=886.0