Looks okay here. What browser are you using MWB?
I like it. Some suggestions:
I think the score should be based on typing speed and not just number of zombies killed.
I don't like the way it randomly selects a word starting with the first letter you typed. You start typing a word and it selects a different one and it really throws you off. It can be difficult to figure out the other word as well when there are many words on the screen and overlapping. I'd suggest moving the selected word to the top of the z-order so it's not hidden behind other words.
And you need an option to mute the music ;)