What is the format (extension) of that file that you downloaded from your colleague's workspace?
Make sure that the extension of the file should .cptx, .cpvc, or .cptl, then only it can be opened in Captivate 7.
Launch Adobe Captivate 7, then click on file open and then browse that file and open in Captivate.
I suppose you downloaded from acrobat.com because you are talking about workspaces. When I send cptx-files or cptl-files to users, with SendNow (another service of acrobat.com) it happens sometimes that the extension is changed by the OS to 'zip' because all CP-files are in reality zipped folders. But a file with extension zip will not be recognized by Captivate. If you are sure it is a Captivate file (cptx), rename the extension to cptx and try again.
Thanks guys for your replies, which both make sense. In the meantime, on a MacHints forum I found someone with a similar and a simple solution. Use Safari instead of Firefox.
Firefox makes Mac compress the already-compressed files, whereas Safari unzips the download for me.
Thanks again and I hope my update helps future problem-solvers.