To reach Grizzleheim in Wizard101, first reach level 20. You have to be a crowns player or a subscriber, free-to-play wizards are not allowed to travel to Grizzleheim except for the quest in Olde Town. When you hit level 20, Merle Ambrose will call you to his office and gives you a quest to go speak to Bartleby and you will then be able to enter Bartleby and use the spiral door. Then you go to the spiral door, open it, then choose Grizzleheim and there ya go! You are in the land of warriors and bears. :)
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You would have needed to talk to Baldur Goldpaws in Olde Town at about level 3, or below. Higher is optional, but I recommend doing it right away, or before you gain level 20, which is when you unlock GH. Merle will summon you to his office to explore the lands of bears.