Select products for an order via check boxes in Access 2010
You have an order/order detail set-up and this means the easiest way is to create an order form with a detail subform. The subform should be based on the table Products illustrated above, with a combo box for PID drawing the row source from Lookup products. The link child and master fields will be Order ID. 

You can see something like this illustrated in the Northwind sample file. The sample files should not be followed exactly, they are only illustrations. In particular, the code is inclined to be a little dodgy.