No so simple darknate.
Its true that one needs to add a single sourcenat rule as your link describes at the top of the source nat chain,
but the not necessarily so for the associated DSTNAT rule.
The dstnat rule depends upon if the ISP connection is a static WANIP or a dynamic WANIP.
Quite correct if its a static WANIP then only the source nat rule is required, but in a dynamic WAN IP scenario there are at least 3 options..
And thus I prefer this link to point readers too. (no bias)
http://forum.mikrotik.com/t/forward-external-ip-address-of-router-port-22-to-internal-machine/148943/3
What I would like rextended to do is expand upon his method as its not so obvious for us mere peons learning RoS!