I am almost positive there are no buses from one theme park to another. The monorail can get you from Epcot to MK and vice versa. To park hop from some of the parks (like AK to MK), I believe you have to go from a hotel to a theme park. Hence, the PP's suggestion to take the CR bus from the AK to CR and then walk to the MK.
There are buses from the AK Lodge to the MK (just in case you meant the lodge and not the park).