Well, the problem is the way OSX replaces a folder if you drop a folder of the same name on top of it. If you expect it to merge, you will be highly disappointed:O I never even knew windows could do that (and I was a windows user since 3.1)...so, I certainly don't miss it being gone with OS X. There are actually very good reasons it happens that way- Mac is cheating by using special folder hierarchies to provide package-like functionality for Applications and a handful of other magic things. If the merge occurred like it apparently does in windows, you would have to delete your app first, and then drop the new on in place.

I don't like StuffIt...lots of annoyances in that little app. For unarchiving, I use BetterZip (http://macitbetter.com/) which has many great little time saving features. It remembers the last several directories (or is that OS X that does that...not sure) so, I can work through several new installations in just a couple of minutes. It basically works the same as winzip.

I would look around for a tutorial on installing poser content with Macs and just point your users to that. The odds are pretty good that folks using your product will have already installed zips before, and won't actually need instructions on how to use them, but, in the event they haven't, you can rely on the community to get it exactly right without having to worry them with warnings like "Doing a PC type install by extracting the zip file in your main Poser folder (or other place) may result in serious loss of data on Macintosh systems"