chk woodruff or square keys in axle to rim connection - they sometimes fall out during tire change/repair...

Well, a 608 should have 2 traction belts. Did you use the OEM belt? MTD wants you to replace both traction belts at once. In my experience, one can get away replacing a rear only, but not a front only. With all that being said, if it doesn't move at all, possibly the bearings in the VS (variable speed) pulley are tight. If that were the case, the front belt would smoke when the pedal is pushed down. If this is not the case, you will need to confirm that the transmission pulley is indeed turning when the pedal is depressed. If that is the case and it still doesn't drive, more than likely there is an internal transaxle issue. rcbe does make a good point, but I would think that your rider has the "double D" shaft rather than the key. Please let us know what you find. If you need more help, it is best to have the complete model # from below the seat.