First, follow the campus directions to reach UCI, and check the campus map. You're headed for Donald Bren Hall (building 314, grid F6 on the map) and the closest lot is 12B. (Note: Be sure to look for Donald Bren Hall, not the Bren Event Center, which is on the other side of campus.)
If you're coming from the north, your best bet is to take the 405 to the 73, then take 73 south; get off at Bison, which is the last exit before it becomes a toll road. At the top of the off-ramp, take a left onto Bison. Follow Bison onto the campus, and take the second right onto East Peltason. The entrance to lot 12B the first left turn after Bison, just after you crest the hill (there's a left-turn lane, but no light or stop sign.)
If you're coming from the south on the 73, get off at the Bison exit, and take a right onto Bison and onto the campus, then follow the directions above.
If you're coming from the south on the 405, then get off at Culver. Turn left onto Culver and follow it until you reach Campus (after University and Harvard). Take a right onto Campus, and, at the second (I think) light, a left onto East Peltason. Follow East Peltason around to the south side of the campus; the entrance to lot 12B will be on your right after a stop sign at Los Trancos.
On your right as you enter lot 12B, there are around ten "pay and display" spots for which you can purchase a ticket at the blue machine. The majority of lot 12B is a permit-only lot, though, so you'll proabbly need to get a parking permit. You can pick one up from the any of the parking structures. If I've arranged a permit for you, then you should just be able to give your name and they'll give you one. The closest parking structure is the "Engineering Parking Structure" on East Peltason between Gabrielino and Anteater (marked ICS/ENG on the campus map, E7). The map shows the entrance at Gabrielino, although I think it's actually at Anteater. So, if you're coming from Bison, you pass lot 12B, pick up your permit from the structure, and then double back.
Be sure to get a "reserved" parking permit, which will allow you to park in spaces marked "AR"; lot 12B requires AR permits.
Park in lot 12B, and walk towards the bottom right corner (reckoning from the entrance.) Turn right to walk along the footpath (the "ring mall").
The corner of the parking lot you're looking for looks like this:
And then the route to follow looks like this:
That's Bren Hall that you can see through the trees; as you approach, the shade on the sixth-floor patio is pretty distinctive. It's the first large building directly on the path you're following:
The part of the building you can see here is one of two arms that form an "L" shape, pointing away from you as you come from this direction. The entrance to the building is at the inside of the L. As you come past the first wing of the building, you should turn right. You'll see a glass-covered stairway, like this:
The entrance to the building is a door at the base of this stairway. Go through the doorway and into the lobby on the right. Turn left to find the elevators (they will be on the left again) and take one to the fifth floor.
Turn right on exiting the elevator (it's the only way to go) and then immediate left down the corridor. Take the next left and follow the corridor; my office is 5086, which will be on your right, a couple of doors after the corridor makes another left turn.