You can actually see Freke's cell from where you warp in at the 3-1 archstone. Look straight ahead, and you'll see a glowing light. That's his cell. As you get closer, you'll hear the ball of light humming, like a magical engine.
From the archstone, turn right, go upstairs, go to the other end of the corridor, through the two gates with the Octopus-headed Wardens, take a left at the singing lady, go down the corridor, down the stairs (one floor), and you'll arrive at Freke's locked cell (the third one).
To get the key, you'll have to go up the tower on the right (where the Dregling is running the magic machine for Fool's Idol). You'll find it hanging on the wall in the very back of the hallway.