To add a stock item to a document simply double-click on it (in the stock item list) and it will appear as a line item in your proposal or invoice, with a quantity of 1. If you are only selling 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 selling a larger number, then add one by double-clicking, then double-click on the item in the proposal / invoice'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 document. (You may also access this by right-clicking and selecting "Edit this item" from the pop-up menu.)
To add a special, one-time item, simply right-click anywhere in the proposal / invoice's list, and select "Add one time item" from the pop-up menu. Fill in the quantity, cost, description, etc., and click OK.
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.