Hello!
I'm enjoying Dawnguard with my already werewolf archmage, but I don't see his perks tree at all! How can I activate that tree? I was werewolf already before the release of the Dawnguard DLC.

Hit the tab or / key while in werewolf form to pop this perk tree.
It works the same way for the Vampire Lord.

Regardless of system I think it's whatever button you would usually hit to look at perks, but specifically while in the beast form.