Spoiler:
Run to the end, taking the high path doesn't get you there fast enough. Grab the rope at the end and get in the very middle, too high and the piranhas will kill you, too low and it won't work. (EDIT: Sorry, I said Piranhas because I meant to give those an off-color mini-piranha GFX, but then didn't. Heh ^^;; I will in the final version, though.) By being in the very middle you'll get forced off of the rope by a gray block. This for whatever reason makes everything blank work like chain mesh. Climb down to the ground and run to the left, all the way back to the beginning. Jump and climb up the air and go over the area you started at, but don't touch the rope hanging there. Fall down and then grab ahold near the bottom, and then climb to the side under the rope until you get to the key. Grab it, jump up above the rope, and then run to the end *again*. Throw the key straight up, jump, and start climbing on air. Hold the run/'hold button and the key will fall onto you and you'll grab it while climbing. Then climb over to the keyhole.