I'm having a lot of trouble getting them to show up.
I don't use the decal tool because it's not grid-friendly like I'm looking for, I create the infodecal entities by hand and put them into position manually, where they end up assuming appropriate shape and sticking to the wall.
In the editor, they show up fine except for the fact that they're always horizontally backwards.
In game, they don't always show, and I can't even pin down something similar about them. Any that I place in my own maps don't show, but I downloaded the sample maps from the level transition tutorial, and no matter which one I load first (from the compiler either way), the MAP 1 sign doesn't show, but the MAP 2 sign does. I've checked the properties and they all match except for the texture property.