There's pretty large signs right outside the
Disneyland hotel with arrows pointing to the Happy 15 entrance. It's to the right when you face the park from the hotel. At Disneysea it's the far left lane at the South entrance. You may need to ask a CM because this one is a bit harder to find.
At Disneyland, you gain entrance 30 minutes before park opening while at DisneySea, you gain entrance 15 minutes before park opening. On extremely busy days, DisneySea may let Happy 15 in early. At Disneyland, you have access to Astro Blasters but I have seen Monsters Inc open for Happy 15 on busy days. At DisneySea, it's Toy Story Mania and Tower of Terror but that may switch to Soaring.
If you just want to ride something and grab a FP before the main guests, arriving shortly before they let Happy 15 in will be fine. If you want the best spots for shows or want to take pictures with Mickey, Minnie, Duffy or ShellieMay without that cutting in to the main part of your day then you should arrive early.