How do I wire a brand new cooktop that has no white neutral wire?
In this case, you would simply not hook up the neutral wire.  Instead you can just put a wirenut on it and tuck it neatly into the electrical box.

Typically 240V appliances require the neutral wire so that they can run the electronics at 120v or provide a plug on the appliance.  In this case, these devices are hooked up to one leg of the hot and the neutral, giving 120V.  

If no neutral is required, then the device can only operate on 240V/208V, though at 208V you should expect the oven to take longer to heat up.

What it means by "Connect only to a 3-wire, 120/240-volt power supply; the neutral conductor is not required for the operation of the appliance,"  is that the 240V needs to come from two legs of 120V service; this device won't function on 240V mains like you'd find in Europe.