Rabbits will, by choice, always (or almost always) use the same area for their toilet.  I suggest letting a new rabbit have a few days in it's new cage to select an area.  When they begin using one area more than the others, put a litter pan in that corner with a few droppings in the pan.  You can also put a paper towel in the tray at the bottom of the cage before putting the litter pan in and then take the wet towel and put it in the litter pan to give them the idea that the litter pan is where to go.

Like dogs, some rabbits get the hang of it quicker than others.  Does seem to prefer to have 2 litter pans, one to use for a potty and one to either sleep in or just to keep clean (presumably for the litter of babies she thinks she will have).

Pelleted litter seems to work better with rabbits than cat litter.  With cat litter they will track the litter around the house in the fur on their foot pads.   The pelleted litters are more absorbent and reduce amonia odors.  Please sure to change the litter frequently.

Hanging a hay feeder one the side of the cage above the litter pan will encourage use of the litter pan as well.  Many rabbits will potty while eating and the hay feeder encourages them to the proper spot.  I do not recommend putting hay IN the litter pan.  They would then be eating soiled hay and could infect themselves with shed parasites.