If you have never been before, I suggest using something like google maps and searching out MK17 9WA. On arrival at Woburn, look for signs (or follow the car in front) to the top of the hill where you will be directed in.
Alternatively, please use the directions provided by Woburn Estates, on their website
http://www.woburnabbey.co.uk/visitor_information/how_to_find_us.php