The Hollywood Sewers are accessible from one of two locations, the first and easiest being Hotel California (such a lovely place), you will have to fight your way through junkies. Just talk to the receptionist until she attacks you, then clear the place out. The sewer entrance is in the back. The second entrance is in Heidi Hollander's secret bedroom. To get here, break into her office (warning, unless you've ousted Heidi from Hollywood this may or may not aggro her unintentionally) if you allow the explosive trap on her door to go off instead of disarming it, it will kill the lone body guard on the other side of the door, negating the need to fight. Once inside you will find a bust with a switch to open the hidden panel leading to her bedroom, and her private sewer entrance.
Once inside the sewer, kill your way around the map, making your way to the south-west most corridor. It is here you will find a computer with an ominous red button. Push it, and the achievement should be yours, as well as the location "Irvine" is now available to travel to on the world map.