The Ithildin symbols show up on your compass and mini-map in-game as a white icon that looks like a U with 3 downward-pointing blades underneath it, kind of. You can simply use the mini-map once you have the forge towers activated and find them that way.
Once you are near the symbol, press
to enter Wraith mode and then
when near the symbol to pick it up.
If you still need help, there's a guide up at Games Radar, which has been reliable for other such "find the X" type of achievements. If you have trouble locating a symbol once you're in the area revealed by the mini-map, this can help:
http://www.gamesradar.com/middle-earth-shadow-mordor-ithildi...Happy hunting!