OK, it seems there's contradictory information at Pearl's site. One part says the sound pack is included in the Studio One Artist download from PreSonus, but there's a different part of the site that provides a link to download it.
Anyway, my problem seems to be all fixed now, and the people at Pearl have been notified about the contradictory language.
For anyone else wondering, there's a download section when you first go to pearlmalletstation.com that indicates you don't need to download the sound pack separately. But if you *log in* on that site and go the the subsequent download section on the screen you end up at when you log in, there's a link for downloading the sound pack. Hopefully Pearl will clear up the confusion on their end.