Garages and Carports - Side door on garage leaking

Just bought this house and there was a new prehung installed in the garage. (side door). During rain water seaps in around the bottom of the theshold. The theshold sites flat on the garage side, but on the sidewalk side this is a big gape (3/4")
My gut feeling is seal this with some cement patch. But the theshold seems to sit on the cement inside the garage nice and flat. There is a little ridge in the cement under the theshold. Is this suppose to keep the water out? This prehung door has the theshold attached to it, so I can't get to the bottom without removing the door. The question is can I build up the cement on the outside or is the ridge set up to allow ater leave if the garage floods or should it be tight. This ridge I speak of ends about an inch short on covering the theshold gap. This is where the water is getting in.Hope this makes sense.
thanks,J

My guess is that the original door sealed against the ridge on the slanted cement saddle (what you call the threshold). Now that there is a pre hung door with it's own saddle, it's at the wrong angle & that's why you have a gap. Is the ground outside the door pitched away from the door? Is there a rein gutter on that side of the garage? Is a puddle forming during a heavy rain?