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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.

Unless the file sender made an effort to reassign all the xrefs full paths for you, and assuming they sent you all the files in the first place, you are going to have to do the manual fix of reassigning them in the top file yourself.

Unless you want to drop all files (XREF and top file) into one single folder, then AutoCAD will find and load them all for you automatically.