'The Man in Seat 61' is invaluable for anybody wanting to travel by train and you'll find all sorts of info on travelling in France by train here:
http://www.seat61.com/France-trains.htm#.URdiEHwgGSM
But a quick check suggests that you can't get a direct train - you'll have to go via Paris with the train from Metz arriving at Gare de l'Est and the train for Disneyland leaving from the Gare de Lyon but that shouldn't be too difficult via the Metro. The problem is the same as for visitors to the UK traveling from, for example, Birmingham and wanting to go to Kent - they arrive at one terminus and have to leave from another because there are no/few train lines that transit through the city. Blame the 19C developers of our train system - I assume the same happened in France.
I suggest that you have a look at:
http://www.raileurope.co.uk/Default.aspx?tabid=39
Rail Europe is the UK office of SNCF and the above website is in English and reasonably easy to use. However, you can't book a direct ticket on the website because of the Metro transfer - you would have to book Metz-Paris and then Paris- Marne la Valle as two separate journeys. So it might be easier to ring them and speak to a real person. Or if you live in the London area, actually go to their offices and buy the tickets over the counter. Full details are on their website at the link above.
SM