Here is a site that shows the valve and a solution:
http://www.kwikkleen.net/easytodo.htm
The threads on the plastic are to attach a hose to run the dirty water somewhere other than on the floor. After a hose is connected to the threads, grab the large plastic collar around the threads and move your hands to your left in a normal unscrewing motion. The valve will open until it hits a stopping feeling.
(The picture on the above site shows a wrench on the pipe behind the collar to remove the valve after it has been opened all the way.- Do this only after the heater gas is off and the tank is completely drained – if and when you are installing their device.)