Arriving from Paris :

Take metro line 1 in the direction of La Défense and leave at Grande Arche La Défense.

alternatively

Take RER line A and leave the train at La Défense.

Leave the metro/RER station at the DOME/B exit via 2 escalators leading up to ground level, opposite the Grande Arche. Take the flight of wide concrete steps to your left then the first right off the steps over a footbridge leading to the black glass Tour Factofrance, the first building on the right.