Here's a video (not mine). You can customize a dinosaur in the Visitor's Center on the left side of the map. When you're walking around the Jurassic Park and come across a paddock (large flat blue area to spawn in a dinosaur), just chose one of your customized ones.
Update: As MissJaded has pointed out to me, you'll need 40 gold bricks in order to do use the paddock. You'll also need to heal a sick dinosaur.