just head down, into the sewer, now just follow the hallway north. Go to your left, and you will see a little glint where the key is, if you already don't see it.
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Assuming that you used the bucket of water with the drain, the key should be here in the mud http://img502.imageshack.us/img502/6241/varrocksewers.jpg
bucket of water
you go into the kitchen sewers left of varrok palace and look there its different for each player it should say rusty key
The key is stuck down a drain behind the kitchen in the castle. Go get a bucket or jug and fill it with water at any water source and the sink in the kitchen gives you water (if you did not bring a bucket, up the stairs there is a rooms at the corner and you can get a bucket in there). Use the bucket of water on the drain and the key will be washed into the sewers. Enter the Varrock sewers using the manhole located immediately east of the castle's keep. Follow the north-western path until you get to a stream and go to the small corner. Search for a rusty key on the ground; it will not appear on the minimap. Grab it.
If you're talking about during the quest Demon Slayer, then what you do is use a bucket full of water with the drain, causing the key to dislodge and go into the sewers. Then go near the Church, and there should be a manhole. go down into the sewers, there should be two hallways. go through the hallway to the left, then turn left when you get to the water, and behind a broken wall, with (i believe) skulls piled up, there the key will be :) hope that helps!!!