RER A is the easiest way to get from Paris to DLP. Just make sure you get on one that says Marne-la-Vallee Chessy (should also have a Mickey Head on most signage), as RER has 2 endpoints in that direction. RER A stops at Charles de Gaulle Etoile, which is pretty close to Arc de Triomphe. The final stop on that line basically dumps you into DLP.