You can pick up a reserved permit 2 days before the date you start the hike. I think when you get your online reservation, it will have details about the no-show deadline, which I believe is 10 AM of the hike start date. I couldn't find it anywhere on the link Bob provided above.
It may also be possible to pick up the permit in the night-drop box at the Lone Pine visitor center, but you should call the Wilderness Permit number a day in advance for that.
Get your Kearsarge Pass permit along with the Trail Crest Exit at the earliest possible minute, since the exit permits go fast. Try going through the steps on the website before the actual date you need, so you know what to look for.