To get the equipment to go under the racks, you should change the Map Diagram Hierarchy (parent) field for all equipment types. The value in the parent field must match the blue key field value of a rack in the project. All object names should be unique.
To automatically bring in backgrounds, you have to
1) Create a background field in the database (you can just call it 'bkg').
2) Populate that field with the path on your system to the background file (e.g. c:\backgrounds\rack.wmf) for each rack record in the table. To create the background file, you can add a huge rack to a diagram and then select Diagram/Save as Graphic.
3) Connect that field to the rack type in netViz (and hide it if you want).
4) Rename the field (that appears in the Inspector) in netViz for the rack type to $bkg.
5) Run a database refresh; all racks should have the rack background under them now..