Where did you find the mandrake root on poptropica?

It is above the high wall to the right on the exterior of the castle (right of the moat before you enter the castle proper).

You need the crossbow from the Armory inside the castle. Once you have lighted the 4 chandeliers in the Library (using the candles), go right to find the glass eye on the floor. Put it in the tiger's head above the left bookcase. This will give you the armory key. Go in and get the crossbow.

Use the crowbar to break the chains on the right side door of the Library. Go outside and left to leave the front of the castle. To the right of the doorway is a tree and a tall wall like the one in the Armory attic. Shoot a few arrows into the tall wall to the right. Before the arrows fall out, climb up them to reach the mandrake plant.