Cod 4

I'm new to gaming and I'm playing COD 4 on my MBP. I'm at the part of the game where you are trying to snipe the character Imran Zakheav. When you are using the sniper rifle it tells you to zoom in and out on the sight using the backwards/forwards keys. I'm using the keys in the bottom right corner of my keyboard, is this correct? When is use these keys nothing happens. Cane somebody please help me out. Many Thanks

 

Doh!...Many thanks