You don't climb it. You have to find the cannon starting kit and explosives barrel on that island and push the TNT under the yellow tooth of the skull, fire the nearbye cannon and the map piece will be where the tooth was.
do you mean skullduggery??
You don't!
it is the skull with the cavity on it.
There is no key card in skullduggery island.
its the teeth on the skull
First, you go to the island due west of the main island. Climb up onto the dock, and past the pirate outpost sign. Go past the trading post, and climb the large skull. Go up the chain an voila!
you do.
Keep going left past it and jump on the boxes and things and find your way up from there.
you have to get canon powder and light the canon the powder is in the shop on that island
On each island once you complete the missions for them.
Its simple, you just have to look up the cheats and do it yourself!
Swim left from the trading area, beneath the giant stone skull. One cannon is at the far left and you can climb up from there. To fire a cannon, you need the Cannon Starter Kit (fuse) from the very top of the outpost's mast.