Hey man I had that problem to but this is what blizzard told me to do.But i already had the WoW installer if you haven't downloaded it before this then all this that i am typing is basically useless crap.
1.)Go to www.worldofwarcraft.com and go to the free trail thing and when you get done with that it(meaning that you downloaded it fully).
2.)After that, you should see a dragon @ the top of the window(i think that you right-click it).
3.)Then after that one some window that says "Already have account" you have to click on that.
Well I hope that that kinda cleared up your problem, if it doesn't i would call blizzard.