To find out how far the downtown Megabus stop is from Orlando transportation (LYNX) I contacted the Travel Orlando; The official source for Orlando Travel Planning. The travel bureau has an online chat that was partially helpful. The following is a cut and paste of my conversation from May 23, 2012.
From LYNX to WDW
He said to take the Fifty Bus to Downtown Disney. The fare is $2 each person, each way. (Fares change so double check with LYNX before arriving.)
Facts about Magic Kingdom