I know this thread is old but there's so much conflicting info that I had to comment.
I believe the best method of waterproofing, especially when building a shower, is with the Kerdi Membrane and Drain system. Dens will work in some applications, but their shower stall installation instructions do not work. There's many ways to install tiles properly, so no point in going into that now.
Another error I noticed above is the suggestion to use pressure treated lumber or plywood. PT should not be used indoors.
Tile 4 You LLC
I've NEVER made a mistake....I thought I did once....but I was WRONG!