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
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.
There's either the tallest mountain in club penguuin that you cant get to or there is none. You can only go to the ski hill or also called ski mountain.. otherwise no idea