Mac OS X Wireless Keyboard Power Button

The power button is on the right hand side, it's part of the "tube" at the back and clearly marked with a power button symbol.

Did you check the batteries as well?

Many people have experienced this problem. Often this was related to bad contacts inside the keyboard. This is a fix proposed by several people:

Fix

I resolved this by rolling some soft household metal sponge [...] into a small ball and dropping it directly on the contact, then jamming in the batteries. Don't worry, the case will close (my ball was about 75mm in diameter). [...] It is nevertheless important that the metal sponge doesn't touch the casing, this could cause a shortcircuit.

source: https://discussions.apple.com/thread/1797139?start=0&tstart=0