Whenever I use the texture for signs, I can't see anything but an empty box. Is this supposed to happen? Signs meaning like the directional arrows on walls and fences.
You should place the signs as overlays, since the sign textures are partially transparent. Texturing a brush with them will make a part of it invisible.
Overlays are basicly like stickers in hammer. Use the overlay tool ( ) with the texture selected and click the surface you want to add it to.