In this dialog there are two lists:

A list of Known Functions - the total list of functions that have been created (including prepackaged functions).

A list of Displayed Functions — functions that are displayed on the toolbar.

You can add to the Known Functions by clicking on the New button and defining a function in the next dialog. You can also select an existing function and change it by clicking the Edit button. If the list is very cluttered you might opt to get rid of functions using the Delete button.

A function won't show up on the toolbar until it is moved to the list of Displayed Functions. This can be done by clicking on the arrow button in the middle, the one between the lists. The order of function buttons can be controlled with the up and down arrows, and you can choose to Remove a function from the displayed list.