Here is a place to start. Just got my 2011 Snapper back together two days ago. Had to replace the leaking oil seal. Firstly, sit down in the seat. Don't start the machine just sit there. Put the shifter leaver in neutral. Now first thing is reach down with you RIGHT hand and push down on that little flippy leaver on the front of the plate with the 1-5 numbers stamped into it. HOLD IT DOWN. Now with your LEFT hand move the mower deck engage handle into the drive position. HOLD IT IN POSITION. Next with your left heel stomp down on the mower deck blade brake pedal. Now make sure you keep that pedal down. Push in the clutch/brake pedal and you should now be able to move the shifter handle anywhere you want. Start the machine now and see what happens. If everything works then you have it. Your actual problem sounds like a bad seat switch. Here is what I did to my machine. Down by where the throttle cable goes into the motor there is a little black plastic block with two black wires. Un hook them. Take your meter and see which one goes to ground when you turn the key switch on and off. Hook that one back up. They rest of the system and safteys are that other black wire. The module goes bad. Do you want to spend around $40.oo on Ebay or $60.oo at a dealer for the module?
This is my humble foolish dangerous not to be tryed at home, out at sea, amature shade tree mechanics, etc etc etc.