You're a minute away from recovering your save files.
The latest patch for the Steam version of The 7th Guest creates a dedicated folder for your save files. Before you can continue your progress in the Windows and Linux versions of that game, you need to move your old save files to that folder:
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Right-click on The 7th Guest in your Steam Library, and click on Properties.
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Click on the Local Files tab, and click on Browse Local Files.
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Drag your "t7g.***" save files to the Saves folder on the same screen.
Because the OS X version of The 7th Guest has been completely replaced in the latest patch, older save files for that version doesn't work with the new version.