View Happy Isles Driving Map in a larger map
Happy Isles is at the far eastern end of Yosemite Valley, next to the Mist Trail - the two trails run parallel to each other on opposite sides of the Merced River. The road that goes past the trailhead is closed to non-park-employee traffic, so you'll have to park elsewhere and either walk to the trail or catch one of the shuttle buses, which are allowed to drive directly to the trailhead.
The parking lots marked on the above map are, from left to right, The Yosemite Village day-use lot (about three miles from the trailhead), the Curry Village lot (a mile from the trailhead), and the trailhead lot (half a mile from the trailhead and ostensibly intended for Half Dome & Mist Trail hikers, though there are no signs indicating this and no one will be asking you where you're going). You'll have an easy, flat walk from any of these lots to the trailhead, but if that doesn't appeal to you, the free shuttle buses stop all over the eastern end of the valley (including at the Yosemite Village & Curry Village parking lots) and will eventually find their way to the Happy Isles trailhead, provided you're on the eastern loop - see this shuttle bus map (PDF format) if you're worried about which bus to get on.