If the button can't be mapped as either a forward or back button(recognized in many games as Mouse4 and Mouse5) so you can use the "Mouse Click Recognition" option, you'll have to map it to a keyboard key using the manufacturer's software.
If you don't want to lose the use of a physical key on the keyboard, you can try mapping a key like F13 to that button; It doesn't physically exist on most keyboards, but it will be recognized by VoiceAttack.
If the manufacturer's software requires you to press a key to map it, you can have VoiceAttack press that key for you(You can click the keyboard icon under "Press Key" in the "Add a Keypress" action and select F13 from the list), though note that not all software will recognize it.