To allow Advance Steel 2013 running on the Advance CAD platform to open dwg files saved with AutoCAD 2013 file format, please set the default save file format in AutoCAD to "AutoCAD 2010 .dwg". This is done by accessing the "Options" dialog in AutoCAD, "Open and save" tab, and changing the value for "Save as" parameter.
Also, please install the Service Pack 2 for Advance Steel 2013 to allow AS + AutoCAD 2013/2014 to create drawings using the saved file format from the AutoCAD options dialog.
For existing drawings, you can "convert" them by opening with AS running on AutoCAD 2013/2014, and using "Save as" option to save them with the AutoCAD 2010 file format, allowing Advance Steel 2013 + Advance CAD to open them correctly.