From the airport, take I-5 North.
Exit on Gilman Ave/La Jolla Colony
Take a left off of the exit on Gilman Dr.
Gilman drive runs directly into the main campus of UCSD. You'll see signs for the medical center there.
While you're there, if you have time, check out Windansea beach. Go in the morning (9:00) or in the afternoon (4:00) if you can, and you'll see what the true local nature of San Diego/La Jolla is (lots of cool people who gather together in a hut in front of the best wave in southern california)
Enjoy. It's the best city in the country