The note can be found in the Sierra Madre casino on the counter of the dining room on the 2nd floor, next to three Sierra Madre chips. It unlocks the security management control terminal upstairs, which is normally locked with hard difficulty, in the casino's security room.
The passcode for the cashier room security hologram is "MrFizzle17." Please don't forget it again, I could lose my job if they found out I gave it to you. See you in the electrical closet after the Gala Event?