There is a DWG viewer released by the companies who work with this type of files. eDrawings has proven to be the industry standard for this operation. Beside a viewer, you will have additional features used to better manipulate the DWG files.
Install the software on your MAC based on the version you use (10.5, 10.6,10.7, etc.) and use the File > Open to import your DWG files into the application.