Hot water heaters such as the Kenmore Power Miser 9, come with a plastic drain valve that is most likely going to start leaking soon. Mine started leaking a few months ago through the plug valve washer, but recently the valve sprung a second leak and was leaking right at the threads so it had to be removed. You will likely have to replace the valve in as little as 7 years, as was the case for me. It is extremely difficult to remove these little bastards without breaking the things right off at the threads. If you break the thing off at the threads you may need to resort to special tools for removing broken bolts, or even perhaps painstakingly using a hacksaw blade to slowly split the threaded end so you can pry it out. Anyway, there is an easy solution, don't break the valve off completely yet with anger! The trick to remove this bastard is to first remove the white/grey knob assembly (by twisting the white part) and valve plug washer... once that is removed, you can then find around your house a metal rod that you can insert into the open hole where the knob/plug assembly used to be... and now with your adjustable wrench and your rod together in each hand, it should turn! You should be able to remove the valve! Even if you cracked the plastic flats slightly trying to remove the valve with your adjustable wrench, this will likely still work like it did for me, I illustrate below.... once it's off I highly recommend replacing the valve with a standard 3/4" NPT brass nipple, full-port ball valve, and garden hose adapter as shown in the last image below. These water heaters all use standard 3/4" NPT fittings which are readily available in your local hardware store, likely even cheaper than the replacement value of the plastic valve from Kenmore, and the brass fittings are a million times better quality and will last much longer. Even if they cost more I still think it's completely worth the upgrade. In fact, if you buy a new water heater in the future, look for the stupid plastic drain valve and immediately replace it with brass fittings. It will make life easier years down the road. They drain faster too.
With your adjustable wrench it is likely you will crack the plastic flats shown above. Do not continue to force it! Use the metal rod method together with the wrench and you'll be good to go...
Above is a steel threaded rod that came from a cheap C-clamp I bought at Walmart. I simply removed the C-clamp's little pressure cap from the ball joint, then unscrewed the rod right out of the clamp. Makes for a great rod.
Grab the metal rod with your left hand, wrench with right hand, and turn counter clockwise. Hopefully this will work for you.
Above is a 3/4" brass nipple that is 4 inches long (for my convenience), a full port brass ball valve, and a brass garden hose adapter fitting. Be sure to use a few wraps of Teflon tape on each of the 3/4" NPT threads prior to assembly. Tighten good and tight. This full port brass fittings arrangement also can drain the water tank much faster. Typically with the OEM valve it will take 30 minutes to drain your tank, with this, it will drain in just a couple of minutes or less, so it will flush debris better too.
* The white cover that went over the valve originally will likely not fit over top any more. This is cosmetic, but also a child safety issue. I recommend you install a some form of child proofing - perhaps a locking cap over the end, or something to lock the valve handle such as a child safety lock or valve lockout device to prevent scalding if a child plays with it.