There are several ways of getting to Martinborough. The most direct way if travelling from Wellington via the Rimutaka Hill is via SH53 which goes from Featherston at the foot of the Rimutaka Hill Road through to Martinborough.
As this is the most direct route it can also be very busy so an alternate route which may take 20 minutes longer but will avoid the usual traffic build up can be found here
Travelling from Greytown you can join either of the above routes as seen here
If you are coming from further north of Carterton the best route is via Ponatahi Road
You should be prepared to spend some time in a queue as there are regularly up to 30,000 people attending the Fair each day.