Grayscale map for transparency?

Does anyone know where I can find a blend file with

  1. a grayscale image packed in and
  2. used as a transparency map (Ztrans and all)

I can’t figure out how to make it work for the life of me.

To the very least, here’s my attempt :
http://www.emonville.com/3D/box_test.blend

Any help is much appreciated.

Fix : Forgot to set Material Buttons-> Material Tab -> A (alpha) to 0.000
Ugh, I didn’t even realize you needed to do that o_o

I don’t have anywhere to host a blend file so instead I’ll give you a tutorial for it.