well, since nobody else is answering, I'll add my little bit of wisdom,
or lack of it:

I don't know anything about this particular tractor, but the
hydrostatic drives GENERALLY have a fill plug somewhere on the
hydraulic tank - the tank may be under the access panel on the floor in
front of the seat. If you're not sure, the plug will probably be a
round plug with a square socket in the top that a 1/2" or 3/8" drive
ratchet will fit. If the plug is not on top, it may be on the side of
the tank and doubles as a level-check hole (when oil reaches the height
of the hole, it's full - much like a rear-end on a rwd truck or car).
Sorry for a lack of specifics, I really don't know this brand of lawn
tractor, but no one else seems to be attempting to help.