Entering a purchase with an inventory item that is set up for actual costing

 We currently are on std. costs.  We set up a new business unit to try to do actual costing.  I am somewhat confused on how the process of actual costing works with a raw material (buy) item.  I have the item defined and the business unit item defined.  I put a price in the default actual cost field.  People Books infers that this is the cost that is used until a PO/receipt is created.  I tried creating a PO using this item.  I receive the message  - The inventory item on Line 1 requires costing configuration.  I tried rolling up