trk001:
This is how I would do it:
1. Select the Door.
2. Activate the Drag command
3. Activate the Snap Point Constraint (Snap Points is set to Half)
4. Click on the two grid lines you want the Door centered on (click anywhere on the grid lines, the Door will stay in the Wall)
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