There is a yanmar forum on TBN under other tractor makes or something. You will get more help there, thats were alll three of us spend time anyway.
I have your tractor and there is a triangular plate on the bottom of the transmission sump. It faces foward on the tractor or the the side on the front side, i cant remember now but think its foward. It has i think 3 bolts on it. You have to drain the fluid and then pull this cover. The filter (more like a fine wire mesh screen) is behind that plate. Pull it out and wash it out in some deisel fuel and then reinstall.