The flat prong goes in flat side facing up. It presses into the housing on the side with the small rectangular opening in the plastic housing, between the flat edge of the molex pin and the housing. Pressing the tool into the housing will flatten the locking pin back against the pin itself.  You should then be able to pull the wire and the pin out of the housing without a lot of pressure being applied. You should only need to pull gently on the wire to release it and the pin from the housing. Sometimes you may need to repeat that process more than once to release it or pull the wire more firmly. Be careful pulling on the wire with too much force, as it can pull out of the pin, which leaves the pin inside the housing and is all the more difficult to remove without the wire to pull on.