While I really think this is a question for a Mac discussion forum, reading below may help
Mac and Root User http://forums.adobe.com/thread/879931
If those previous discussion don't help, you might ask on an Apple forum... file access is at the operating system level
I understand the Apple file sharing queston but fundamentally is what I am
trying to do possible in Elements? i.e. have 1 catalog stored in a shared
location that 2 different users will be able to see (but obviously not at
the same time)? Elements must have been architected with a requirement to
support sharing or not of a catalog.
If I can find out if it is technically possible in the product then I know
the issue must be in the Apple layer (or there is a bug).
As long as both accounts have administrative privileges, there is no reason this should not work. (Assuming both of these are accounts are on the same computer, of course.)