Help on Smoke Effect

Hi everybody, this is my first post and I apologize if it’s in the wrong place.
Im following a tutorial of Blender Cookie called “Dissolving Logos Into Smoke With Blender”. Everything was going fine, but when I rendered the scene it showed an opaque box containing what I believe is the space where the smoke has expanded. How do I fix this?
Here is the link to the .blend file:
Thanks a lot in advance!

For the domain object : in the Materials tab, set Density to 0 and in the texture tab, turn on Density.