On the A61, take the exit of the Borussia-Park and follow the signs. If we may advise you: leave your car at P7, the car park the closest to the motorway. It's a little bit further to walk to the stadium, but after the match it can save you hours of waiting.
By public transport, you can take a shuttle bus on the train station of Mönchengladbach. And with your match ticket, the buses are even for free.