To add a stock item to a purchase order simply double-click on it (in the stock item list) and it will appear as a line item in your purchase order, with a quantity of 1. If you are only buying a few of this item, you can increase the quantity by double clicking again (each double-click in the stock item list adds 1 item above).
If you are buying a larger number, then add one by double-clicking, then double-click on the item in the Purchase Order's list. This will bring up the line item editor, where you can type in any quantity, change the unit cost, modify the description, change the unit of measure, or delete the item from that order. (You may also access this by right-clicking and selecting "Edit this item" from the pop-up menu.)
Sometimes you may be quoted a lump-sum figure on a large quantity of something (x number of lineal feet, x number of gallons, etc.); but you don't know (nor care) what the unit cost is. However, the program needs to know in order to add things up correctly. Click the [?] button next to the unit cost figure to show the unit cost calculator:
Note that whenever you add or change items, the extended pricing and order total are instantly adjusted.
Also note that when you select an item for editing you can make it's description MUCH longer than is allowed in the stock items list.
To add a special, one-time item to an order, simply right-click anywhere in the Purchase Order's list, and select "Add one time item" from the pop-up menu. Fill in the quantity, cost, description, etc., and click OK.
If you have added a one-time item to an order and then realize that it ought to be a stock item, right click it on the order's listing and select "Add to stock item list".