Staffan:
When you open the DWG file, there is a Translator selected at the bottom of the dialog box. Click the
Settings... button, go to the Settings in Selected Translator panel >
Drawing Unit and change the
Value of 1 DXF/DWG Drawing Unit in ArchiCAD option.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David