Standing in the large atrium, between the entrances to the security checkpoints, you will see a bank of elevators (well, the glass walls of elevators whose doors face away from you). On the right side of the elevators is an escalator that will take you up to the second floor front desk. (The down escalator is on the left of the elevators.)
The elevators work only with your room key to prevent casual visitors to the upper floors of the hotel. You may be able to take the elevators from the lobby up to the front desk -- we always use the escalator.