To assign a key to an item:
1. In the "Keyboard" tab from the maintenance screen
2. Press the "Buttons Types" dropdown (it will display "System Keys" to start with). Select "Items".
3. Press the "Available Keys" dropdown, and select the item you want to assign a key for.
4. Press a blank key on the keyboard (you can scroll the keyboard by dragging it around). That will place the the item on the keyboard.
5. To sell then item, all you have to do is press it from the main register screen. You can then finalize the transaction with a tender key, like [Cash], or [Check].