This time, you're going to have to sneak past that guard. He's pretty vigilant, and Rosa won't crawl, so it'll take some doing.

Did you notice the key rack on the end of his desk? There's one key there labeled "fuse box."

Joey can help you get the key. Have you noticed that when he gets close to the guard, the radio goes on the fritz? And when that happens, the guard fusses with the radio?

Stand next to the key rack and wait for Joey to interfere with the radio, then quick grab the key off the rack.

Okay, halfway there. Go back into the lobby by the elevator and open the fuse box on the left wall. (You don't need to actually use the key on the box; if you have it in inventory, Rosa will just use it herself.)

Save your game (because you won't get another chance for a long time), then click on the fuses to knock out the lights. Now you need a place to hide while the guard comes over to check the fuses.

Dash over to the phone alcove to the right of the elevator and stand there until the guard walks by. Once he's gone, slip into the hall and go see Susan. You need to be quick, so you may have to try this a couple of times before it works, but it does work, I promise.