As mentioned before there doesn't look like a lot of room on these mowers for extra weight Norm ,possibly you could fit a bag of sand or weight behind the seat.
I guess I would get a bag of sand and put it on the seat or behind it and see if that fixes the problem and then you know how much weight to add and if it doesn't work you haven't wasted much time.
I'm sure you could make a bracket mount from somewhere to add a weight .
The easiest thing may be to just put a metal Diamond Plate/Checker Plate on the concrete where it slips then put the plate away when the mower isn't being used.
