Like (I think) most here I'm not a mac guru. Ther's a discussion group
focussed on mac / office which may be a good place to ask (as well)

http://www.microsoft.com/communities/newsgroups/list/en-us/default.aspx?dg=micro
soft.public.mac.office.powerpoint&cat=en_us_daf6a0da-8c15-478b-bdef-02f74bb0ec8a
&lang=en&cr=us

fyi  all --

I found a solution to this problem. There may be others, but this
works.

As many others have noted here, the issue probably doesn't arise if all
of the files you link to are in the same folder as the PPT, but that
was not an option in my case. All my linked files were in subfolders
with relative links.

The problem appears to be that files saved in Windows lack information
(resource fork??) that the Mac needs in some of its file system
operations.

I got an external drive formatted for Mac and installed the program
"MacOpener" from Dataviz on my PC. MacOpener not only reads Mac drives,
but also writes files to it in native Mac format. Then I saved all of
the PPTs to the drive on my PC (can't just copy existing files; have to
open in PPT and "save as").  Going forward I will just do all my PPT
work on this drive & not have to take the extra step.

These files now work fine when moved to a Mac.

A CD, of course, must be a dual-format CD. Mac can read a PC CD, but
the links will break again. I use Toast on the Mac.

Mike