Access to the Magny-Cours circuit is very easy thanks to the motorway network between Paris and Nevers which was specifcally built to provide spectators a convenient travel to the grand prix.
If you do not want to take the 250 km drive from Paris there are several rail links that bring you to the Nevers station. The trains are backed up by a shuttle service during the French Grand Prix weekend which will take you to the circuit entrance (a return ticket will cost €10 ).