Of course first you'll need to be able to go into the sewers. As you're asking for a full solution, spoilers ahead...
... For the key pad, press random numbers, and then when the alarms start to beep, run away a bit, but make sure you've got your clairvoyance handy. When the G-man comes out to put in the correct code, use claivoyance on him and you'll get to see him put the code in. Remember the code and then enter it yourself to enter the building. The rest should be smooth sailing.