I believe section 1.2.1 Sunlight of the tutorial covers this.

ok you have to emit photons to have global illimination so whatever your light souce is you have to select it and go to the attribute editor and under the Mental Ray tab check on Emit Photons, you may have to tweek the amount of photons to get the desired result so this can envolve a bit of time and many check renders.
If you do not have any Lights you can create one and have it emit photons only by turning off it's intensity.