Peter:
To open more than one object, place them in plan, select them, then use the
Open Object... command, you will get a dialog box asking if you want to open the objects. I have a .pln file with all the objects in a library placed in plan, so selecting and opening a group of objects for editing is simple.
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