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Hi Heidi,

Can you explain what is shown on the attached screenshot?
Is it extracted contents of ZIP archive in Finder? I can suspect these files are inside Dropbox folder and red cross means (most likely) that file names contain an invalid character(s):
https://help.dropbox.com/installs-integrations/sync-uploads/files-not-syncing

Indeed, look at forward slash in the file names - OS X restricts the use of forward slash characters in file names.
That is why these filed cannot be loaded as Xrefs inAC for Mac. You need to ask your client to rename them and correct Xrefs paths or you have to do this yourself.