It is just the same as in Diamond and Pearl, which is pretty simple:
First, you must have defeated your friend who meets up with you at the bridge of Canalave City. Once beaten, you may cross to the other side of the town. You may need to defeat the Gym Leader there, who is a Steel type, convenient if your starter was a Chimchar. After that you may go to the "bridge", which is under the Gym. Talk to the sailor in there, and he will take you to Iron Island.
The same sailor will take you to two differant islands that allow capture of Darkrai and Cresselia, but that you need an event item for, and the event passed ages ago, so the sailor will just be your captain of the ship to Iron Island.
Hope this helps :D (I havent played my Plantinum for a while :|)