We took the bus from Nîmes rail station to Pont du Gard. Unfortunately nobody knew which bus to take, so we had to wait until the bus was due to leave (the tourist bureau had told us 11am). We eventually saw a bus with a destination sign of "Gard" and another town. The bus driver was very helpful, realising we were tourists. He asked if we wanted to go to Pont du Gard, "aller-retour" (return ticket). It cost about 11€ each for a return ticket. When we arrived, he told us what times the return buses came, and from which stop.
Be careful because the location is rather remote and the buses are very infrequent - you could be stuck there for hours in the dark if you miss a bus.
The trip took about 40 minutes, in a slightly old coach-like bus, and it's a reasonably scenic ride through rustic villages. Well worth the experience!