You first go to the two bridges leading to Castellia City and go on yhe right sided one and watch the fence. Next there will be a small gap (and if I remember correctly, the gap will be by a guy in a purple shirt). Go into the gap. There will be lots of dark tall grass. Keep going east until you see a cave thingy formed by trees. Go inside the cave thingy and you will find Virizion.
NOTE: you will need to have seen Cobolion to get to Virision.