Just open the drawing like you would any other normal file. ArchiCAD reads AutoCAD dwg files without a problem. Depending on what template your using in ArchiCAD ( metric or Imperial, for example) it will ask you to set the import settings to do with layers, scaling, line weights, colours and that sort of thing, or you could just use the default template and check the drawings to see that the scale is correct.
Alternatively you can import the AutoCAD drawing into ArchiCAD as an X-Ref, just the same way you use X-Refs in AutoCAD.
