I think what people were suggesting was that you export from SW in ACIS into an AutoCAD product, then export in AutoCAD as 3D .dwg. Opening the ACIS file in SW doesn't get you anywhere. No matter how you try, SW can't export a 3D .dwg, just 2D. You will have to export Parasolid, ACIS, STEP or IGES. The AutoCAD or other product you are exporting to needs to be able to import one of the above. You may be able to go to an intermediate program that will import from SW and export a 3D .dwg that AutoCAD can use.
I have had more trouble exporting ACIS files than STEP or IGES. If you have access to an AutoCAD product which can import STEP, I would try that as my second choice, with IGES as the third choice.
I agree with Jerry's comments, but it seems strange that solidworks cannot open an ACIS file which it has created.
I should add the only time I have tried exporting an ACIS to an autocad user, they couldn't import the file. The option just wasn't there.