Ok i have used these things before but now i cant make it work.
I spawn a info_overlay, set its brush to a wall on my level.
set what material I want, then place th einfo_overlay against the wall. and nothing.doesnt show the overlay.

any thoughts?

Choose the material using the texture browser then use the overlay application tool (Shift + O I think). It's a lot quicker ;)

still not working, i can now put a info_overlay item right where i need it but i dont "see the texture"

i went back to a map i did ages ago with a load and decided to shove some more on but cant as they dont show up

ok i got it working, i cant place new ones, but i can go back to old maps and grab overlays that are at the same orientation as the wall on the new map, then i can copy them and they work.

i've got the same sort of problem, but the texture shows up after a save/close/open of the map, which is a pain in the arse when the map is big, and takes ages to re-load into hammer. cloning an overlay thats on the same orientation as i need works too.