You can walk - the two stations are at opposite sides of the river, but not very far apart.
More practically (if you have luggage) you can go by bus - number 61 does the trip in 5 minutes. As you...
- take the RER line at the gates of the Disneyland park (that goes only towards Paris) Step down at Chatelet les Halles - change for RER line B (branch B5 towards Mitry-claye, or branch B3 towards...