I have an mtd 5.5hp chipper shredder. Each year the fist start in the spring has been difficult, but persistence has always gotten it running. This year I just can't get it to go. After multiple tries I smell exhaust, but I don't think the engine is turning over anymore than the pull cord causes it to do. The gas doesn't smell, but I still suspect that it may be gelled. I'm about as far from a small engine mechanic as you can get, but I'm still hoping some one can give me some advice on how to get this thing going again.

Your carburetor is likely gummed up, I would change gas, use highest octane, clean carburetor and jets. Be sure petcock not clogged, verify gas making it to carb.
You could put teaspoon of gas in spark plug hole, try cranking, if it runs briefly, you will know it IS a fuel related problem.

Stratmando is probably correct. Remove the carb and clean. Remove the fuel tank and flush with carb cleaner, if it is equipped with a fuel filter, replace the fuel filter. One thing that makes it more difficult to start the chippers is that they have a direct drive to the chipping chute, be sure the chute is clear, ONLY CHECK THE CHUTE WITH THE SPARK PLUG REMOVED FROM THE ENGINE!! Our reps tell us that fuel should only be used for 30 days. I think you will find that the carb is gummed up.