Well as the crow flies they really are not far apart with one North East and one just East of Paris. Car is essentially 30 mins all in (driving round the outside on the A104). Going via the centre will add massively to journey time e.g. RER > RER and would not be under say 1h30.
The LGV Interconnexion Est (the High Speed Line round Paris on which both locations have a station - the CDG one is at Terminal 2) essentially is a connection between the main high speed routes North, East and South of Paris and does for rail what the A104 does. That means a very short but expensive trip as BigCj34 mentions - you have to book for a specific TGV and fares seem to run from EUR25.
Of course there is the direct Eurostar from London and trains from Lille/Brussels to Marne-la-Vallée (where you are heading), but I am guessing a flight into CDG was easier/cheaper from where you are coming from?