Hi folks,
I've been there in April 2002. They give out 10 walk in permits per day for THE FOLLOWING DAY at the ranger-station at milepost 21. Last April the station opened at 9 am. No use in being there to early, no first come first serve. If more than 10 people show up it will be a draw.
They provide information how to get there. The walk from the northern trailhead is appr. 90-100 min. and spectacular alone, not to speak of the wave and its surroundings (corcscrew, Hamburger...take your time and walk around) itself !
Bring good shoes & enough to drink, it can be an oven, but its definitely worth every minute! THe formations are very brittle so please be careful to keep that spot rated to be one of the most beautiful in the world intact!
Good luck!