You can assign effects by pressing the
MEMO button and following the instructions on the HC4500's display.
Alternatively, assuming you are using v6, you can directly map the buttons by going to
CONFIG -> Mappers, choosing the
HC4500 in the drop-down list, finding the button you wish to assign/change and then typing the appropriate action in the box, e.g:
effect 'Brake' active
sampler 1 play_stop