You keep going and once you get passed the gate there will be a potion. pick it up and go
back and he'll open the door for you

It's right at the end. Once you get to the top of the map and there's the big archway that
you need to exit through to end the level, there's a chest on the ground near the
jack-in-a-box and the other box. Unlock the chest in Free Play (because you need Alohamora
which you learn in Y4) and you'll get the Veritaserum. Go back to the wizard and he'll
tell you that there's a neaby door that's unlocked. Go the the door that's shown to you
and walk to the back of the room. There's a conveyor belt. It'll turn on automatically and
things will come off it. Avoid hitting the skulls and flowerpots, or you'll have to start
again. Hit 5 pumpkins (orange skulls) and you'll get the Wizard Hat...
And I'm still having problems with the one in the Prefects' Bathroom (Y4). I've found them
all except the 'Goblin' one, and I've tried working the Goblin Plinth several times and
all I get is a blue stud whenever I try it out???