how can I have them use the same form to enter (multiple) materials within an order and get those entries to the tblOrderDetail? i.e. how to I correlate materials with their parent order? I may be over thinking something here and its simple.
Create a form based on the order detail table; I typically do this display this form in datasheet view. Use a combo box based on your material table & bind the combo box to the material field. Include the unit cost field in the combo box as well. In the after update event of the combo box, you will need to copy the unit cost of the material into the corresponding unit cost control of the subform (which is bound to the unit cost field of the underlying order detail table). This stores the current cost of the material to the order. That way you can increase the cost in the material table without impacting any previous orders. This is one of the only times where copying more that one field from one table to another would be justified. Save the form.
Now open you order form in design view. Drag and drop the form you created above. The order detail form is now considered a subform and if you have your relationships set up, Access will automatically link the record in the main form to the related records in the subform.
I actually create a sample database awhile back for another forum member that illustrates the above. It is attached.
I would derive this via a select query
Yes.