With Dragon Medical One (DMO) open on your machine, navigate to 
> Options > Microphone Buttons. For this to appear correctly, you must have the PowerMic as your chosen microphone in the DragonBar menu.
You can then use the drop-down menus against each button on your PowerMic to change the function.
Once you are happy with your changes, click Apply All or say "apply settings and close" to save your changes and close the Options menu.