Modra is located in Slovakia approx. 25 km north of Bratislava. You can reach the city by car or bus.
Arrival to Bratislava (by train or plane): you can take a bus from Bratislava Bus station (or by taxi).
Arrival to Vienna by plain: you can take a bus directly from the airport to Bratislava Bus station and then to continue to Modra by bus from (or by taxi).
You can find more information on Modra here. (tu bude preklik na: https://en.wikipedia.org/wiki/Modra)