If you are taking the south exit from the airport, I would suggest that you try either the Albertsons or the Super Target on OBT. They are both right off the Greenway exit and then you could jump right back on he Greenway and take it the rest of the way to Disney.
If you do have your heart set on Wal-Mart, the one in Kissimmee is on 192. You would need to get off the Greenway at either OBT or John Young (either would do) and head south to 192. Take 192 West and the Wal-Mart would be on the left hand side. You could then take 192 to Disney...but as others have said, this Wal-Mart is very busy and often very dirty (although they really do try to keep up, they just don't have enough employees for the volume).