It should just be press b and press up, if you can't access it.. I don't know what to do for you

You can only get into it when you are a werewolf

This. You have to be in beast mode to make the tree available. Hircine's werewolf skill tree replaces your inventory screen.

The worst (best) part about you replying to my comment makes it seem like your icon is wanting my dragon xD
Otherwise, if the game doesn't open it up, most likely it's a bug and just need to either restart the system or reload a save.