You'll need to remove some material first before adding a fillet. Cut away that radius first and then you can fillet it.
Steve C
Sketch from whatever plane goes down the middle of your part (Top Plane). Sketch a centerpoint arc at the desired radius and cut using the arc. Make the endpoints of the arc coincident to the sides of the part and make them horizontal to each other. You do not need to close the sketch. Cut through all in both direction and make sure the side to cut is going to cut away material in the correct direction. If you're wrong, just edit the feature and "Flip side to cut".
Sketch from whatever plane goes down the middle of your part (Top Plane). Sketch a centerpoint arc at the desired radius and cut using the arc. Make the endpoints of the arc coincident to the sides of the part and make them horizontal to each other. You do not need to close the sketch. Cut through all in both direction and make sure the side to cut is going to cut away material in the correct direction. If you're wrong, just edit the feature and "Flip side to cut".