You must complete at least one quest in this zone before Gerbi Frostfoot (Frostfell) will show you what he has to sell.
Quests that Allow Entrance
If you are in the Frostfell Wonderland Village and click on the door before you pick up a quest, nothing will happen! You will get text that says, "Shhh...it's a secret," instead.
Adventure: Any in the Mission Improbable... series - You can choose a heroic or regular version of the quest to complete. It is followed by more quests that take you into the zone.
This article refers to events, personae and activities only present in-game during the annual Frostfell celebration. Frostfell comes to Norrath each year from (roughly) the second week of December to the first week of January.