If it is like my Digital Audio tower the drive sled is held in place by ONE screw near the front of the drive. It may be hiding under the IDE cable connection. Remove that cable and the power plug and you should see the screw.
It is not easy to get a phillips head down in there and in my case the screw had been stripped by the previous owner so I had to drill it out.
I needed to use all sleds for three drives so I ended up 'borrowing' a screw from another unit.
Once the screw is out, the sled pulls forward a little and then tilt it up since it has two little flaps in the rear that slide under metal ridges.