Getting to Montfermeil
Montfermeil, once a small town in the vicinity of Paris, has now become part of the banlieue, or suburbs. As such, it's accessible through the RER, the light rail system that serves the Paris metropolitan area and is integrated with the Metro.
Montfermeil does not have its own RER station, so you'll have to take Line E of the RER towards Chelles/Gournay and get off at the Villemobile/Le Raincy stop. From there, take the 601 bus and get off at the Paul Bert stop at the intersection of the rue Paul Bert and the rue Henri Barbasse. It's one of the last stops on the 601 line; all of the midway stops are not in Montfermeil but in Clichy-Sous-Bois.
Note: It's also possible to take the RER one station further, to Gagny instead of Villemobile, and take the 604 bus to the rue de l'Eglise stop. This is theoretically closer to the Parc Jean Valjean, but unlike the 601 bus, I've never tried it and I can't say firsthand where it comes out.
At the Paul Bert stop, go down the little rue Grange, which passes by the Allée du Puits (the French word for 'well,' as in 'the well in the wood'), then a church. You should come up on a park, which was once the Parc Jean Valjean and has now been combined with other green spaces to create an arboretum; go down the hill on the road next to the park (chemin de l'Abîme), and at the end of the road, the Fontaine Jean Valjean is on your left.
PDF of the Montfermeil bus routes (1.62mb)