Wiring connections for cooktop range
If the feed only has two wires (not including ground), it's possible that it's only 120 volts (hot, neutral, ground) or 240 volts (hot, hot, ground). If there's only a single pole breaker, then it's probably 120 volts. If there's a double pole breaker, it's probably 240 volts. You can measure it of course, if you have a volt/multimeter.

Since the new cooktop has an additional wire, I'd suspect that it requires a 120/240 volt feed (hot, hot, neutral, ground). This means that you're going to have to pull a new three wire with ground cable, or an additional wire if it's wired through conduit.

If the feed is 240 volts (hot, hot, ground), and you've connected the new cooktop as described (black to black, white to white). You've potentially connected an ungrounded (hot) conductor from the feed, to the grounded (neutral) conductor of the cooktop. This is very bad, and could kill somebody.  

Contact a local licensed Electrician, or do some more research before continuing with this project.