I noticed this thread when recently having trouble with my own decals. Here's an exerpt from my error diary:
--"decals show in the editor but not in-game" ERROR:
Decals turn out to be pretty versatile, but they're touchy. If they don't show up, check you have done these things:
-Decals must be applied to a world brush (if you want them on a brush-based entity, you must apply the decals BEFORE you change the brush to-entity.
-Don't move the brush that has the decals, or try to select the decals themselves to move them--they will not show up after compile.
-if decals are on a sliding door and the door is recessed in a wall, the decals won't be rendered. (making the recessed area hollow and lit should fix this)
***
I realize this is a huge bump, but at least someone searching for this problem will be able to find the answer, and not spend 2 hours themselves figuring why they're not working : P
(This info was not included in the tutorial or anywhere I could find)