To find the trail, you head up from Happy Isles toward the Mist trail. You leave the main trail from the rockfall sign
just past the Happy Isles trailhead sign for the High Sierra Loop/John Muir Trail.
This is what you see looking up from the rockfall sign , which is not exactly an inviting "trail" 8^). Don't go up through here.
Go about 20 feet to the left, and you'll be able to work your way through the rockfall.