Hi,
Really common question. Here are the references:
http://forum.mikrotik.com/t/1-1-nat-dnat-configuration-help/182840/1
http://forum.mikrotik.com/t/quick-guide-bidirectional-1-1-nat-snat-dnat-setup-w-working-example/183024/1
The first one has explanations, he second one has a working example. (The second one will probably get corrected based on my latest comments in the first one.)
Have fun!