From the elevator, look to the left. There are white walls over there and a big 1971 painted above the word "Beta". Put a portal on the wall over there. Now go straight ahead. There is a lobby, with a tantalizing sign that says "elevator to surface", but unfortunately it's boarded up. Turn around and go the way that you came. You'll find a white wall there. Put a portal of the other color on it and go through. Walk to the right and open the door to pump station beta. Look way up on the wall across from where you came in. There is a glowing orange light and a white wall next to it. Put a portal on that wall, then put a portal of the other color on a wall that you can walk into. Go through.
Now look down below. Put a portal on each of the two sections of white floor. Fall into the nearby one. While you are going up through the second portal, shoot a portal of the other color onto the high white wall behind the big gel tube pump things (look to the left of the ledge that you were on a moment ago to see this section of wall). If you did it correctly, you'll end up in an office area.
Go into the office to the right to find two levers that control the gel pumps. Push both levers. Now go through the material emancipation grill and go through the exit door.