To use buttons, hat switches, and axis thresholds to simulate keyboard keys, first disable mapping by turning off the > button in the top left. Then press the button on the input device you want to map. This will select it on the left-hand side of the screen.
If the button wasn't mapped or was mapped to a key press already, the key input field activates and you can press the key you want to use. Otherwise, click on the Press a key item or input field, then press the key.
To change a key without disabling mapping you can choose its entry in the sidebar directly.
If you select the input field by mistake you can press ⌘⎋ or click on the key field to cancel without changing the current setting.
To clear a mapped key select Do nothing or press ⌘⌫ when the input field is selected.
Hold ⌘ and click on the field to enter a raw hexadecimal key code. Key codes are numbers associated with each key on a keyboard. For example, if you need to map something to the Windows Menu key and your keyboard doesn't have one, enter 0x6E here.