First, please check if you are trying to open the drawing on PC with AutoCAD 2013 or other Autocad version. The drawing saved in 2013 version is using 2013 format by default, which cannot be opened by older version of AutoCAD.
If both the PC and Mac are installed with AutoCAD 2013, could you please send me the drawing and I will try on my side? wei.ma@autodesk.com
Thanks,
Wei
Wei Ma
AutoCAD for Mac QA Manager