Use the arrow keys to move, press the m key and have your mouse be hovering over a spot where you want the zombie to be. Then, press the green flag and the zombie(s) will be in the same exact spot where your mouse was hovering over. If you want to remove mobs, press r and it will delete 1 mob. if you want ALL the mobs to be deleted, then press the enter key.
I have Scratch Addons, so yes, you can actually press the enter key to reset mob spawns
