i am using mikrotik V3.2 right now.. before that using mikrotik V.2.9.50
and i am using NAT to create a port forward that allows my client to create a battle-net (DotA) game..
usually you want to set to-ports if you want to redirect to other port like ssh connection is going to port 3333 on your router and that is redirected to your internal ssh servers port 22
if you want that example port 3333 to still reach your server (you changed port to match example) you leave default as macgaiver writes.