I am not able to open a Windows ISIS/Sketch file in ChemDraw/Mac or a Mac ISIS/Sketch file in ChemDraw/Windows.
ChemDraw Mac and Windows can read ISIS/Sketch (TGF). You need to EXPORT from ISIS/Win/Mac (rather than Save As) as a .tgf file. The Export command is typically found in the ISIS/Draw File menu, but see your ISIS documentation for specific details. Also make sure that the ISIS/Mac TGF file has a ".tgf" extension in its name (like test.tgf), so Windows can see the file.