Martin:
One way to see the location of the reference line is to place the cursor over a wall edge and see if it shows the
bold mercedes sign. Another is to use the command
Walls & Beams Reference Lines (as in Andro's screen shot).
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