I open Steam go to my games tab right click L4D2 go properties and i can't find a local files tab.Please help !

I saw some people saying i should verify GCF but i don't know how to do it. I am currently using windows 7 ultimate.Please help !

I have collected all the pieces of the amulet and went to the place where the red mark was. Then I killed the archeopischop and got the wand. What shall i do now ?