How do I enter a supplier invoice for items received?

I am having trouble with supplier invoices.  Here is my scenario/problem:

1) I created a purchase order successfully
2) I received items against the purchase order successfully
3) I open up the "Supplier Invoices" link to enter the bill for items received and do the following:
    - Click on the edit button for "Received items charged on this invoice"
    - The next screen is titled "Select Received Items to Add" and split in 2 parts the top half is titled "Items Charged on this invoice" which is empty and the bottom half titled     
      "Items Received Yet to be Invoiced" which contains the details of the items I've ordered and received.

I can't see any way of selecting the received items and adding them to the supplier invoice.  Is there supposed to be a check box here or something?

There is a screenshot of what I'm looking at here:  www.bigrigrecords.net/temp/fascreenshot.png

Can anyone tell me how to get the items onto the supplier invoice?

Thanks