Rodney,
DMZ would be a NAT rule in the firewall settings. This is where you can configure what internal ip is linked to the public ip address(es).
When it comes to the firewall settings for the router.
INPUT - Would be for all the traffic entering your router. Which i would advise you to use the rules on the bottom of
http://wiki.mikrotik.com/wiki/Securing_your_router
Forward - This would be for all the traffic coming from the internet to your pc's and the other way around. (This is where your LAN traffic would be). So if you want to block ports, p2p, or whatever this is the section in the firewall rules that you would want to setup this.
If you post what kind of things you are trying to do, we can help you more.