There is no bus service from the parks to DTD you have to go via a resort.
Your best bet is to walk/boat/monorial or walk to the resort closest to the park you finish up the day at. For example if your at MK walk or take the monorial to CR and then take the bus to DTD. If your at Epcot walk (or boat) through the IG to the BC and take the bus to DTD. From MGM the closest would proably be the Swan/Dolphin (again boat or walk) and from AK take the bus to AK and then a bus to DTD.
Honestly though this is when dh and I often opt for a cab - it's worth it, JMHO!
HTH
TJ
