I need help with photoshop!?

I'm following along this tutorial: a href="http://psd.tutsplus.com/tutorials-effects/create-a-powerful-mental-wave-explosion-effect/" rel="nofollow"http://psd.tutsplus.com/tutorials-effect…/a I'm on step 10. Now I do everything it tells me to do but I don't get how you mask the parts that it tells me to. I tried coloring with a black brush but that only makes them more black, not disappear. I know I'm doing something wrong because on the example picture it has 3 white x's and I don't have any of them. Can you help please?