If you are talking about the Ski Hill also know as mountain, you can enter it by going to the Ski Village next to the Beach and Dock and click on the mountain like object near the tours stand. Then you will be on the mountain.
If this is 2010 mountain expidition you get there through the gateway in the ski village you go to a survival camp but can only climb the mountain if you are a member.
u have to be a member, and it HAS to be during the festival of flight, which started today. go to the forest and get on the hot air balloon. u will go to the tallest mountain.
You cannot unless the "Mountain Expedition" Party is going on at the moment if it is the mountain expedition party there will simply be a doorway (for members only) to get to the mountain