The Kissimmee bus station is not very far from Disney, maybe a 35 minute Lynx (city bus) ride, or perhaps 15 min by cab.
You will have to arrange your own transportation either way. Once you get offf the Greyhound (or Amtrak which is at the same location) you are on your own.
You would walk to Broadway ave. (if taking the Lynx bus) and take a southbound 4 to the Osceola square mall, then transfer to the 56 that goes to the Transportation ticket center at the Magic Kingdom.
actually it would take 1 hour 25 min by lynx, probably 30 min by cab, (don't know what I was thinkin!) Here is the Lynx bus route:
http://trip1.golynx.com/ntp/en/?#result_mark