You can plan your journey using Rejseplanen.
The address is Roskilde Dyrskue, Darupvej 19, 4000 Roskilde. Travel to Roskilde Station by train and walk to the bus stop Roskilde Station, Sygehuset, Ny Østergade. From here take bus 202A to Roskilde Handelsskole (Maglegårdsvej) and walk for about 13 minutes or take the 212 bus to Dyrskueplads (Darupvej) and walk for about 8 minutes.
If you're going home by public transport, you can take a free shuttle bus from the bus parking area by Entrance East at Darupvej to Roskilde Station. The shuttle bus starts running from the event area to Roskilde Station at 20:00 and will continue running throughout the evening as needed. It is not necessary to book a bus ticket for the shuttle bus.
Please note that you cannot take the shuttle bus in the opposite direction – from Roskilde Station to the party area.