Haynes manual are not a good reference.Take a wrench that it one milimeter smaller than what is required and beat the crap out of it, with a hammer, until it goes on, then you should be able to pull it off. Off course you should get new drain plugs. I used the round circle part of a wrench. You may also need a new wrench. According to Haynes, you should never use a socket wrench to remove a drain plug. This is probably why mine became rounded over time
You can also drill it out like they did with my wife's car at the shop. I blame walmart for rounding the drain plug here.
I always use a socket wrench for the drain plug but I also don't over troque it! I also have the valve like Munky showed, very useful!!!