After you enter the museum, turn to your right and head for the door with notes taped on it. Those are for you.
Hint 2
The office door is locked. Maybe one of the notes tells you where the key is.
Hint 3
The note in black ink says, "Since we don't have a doormat, you'll find your key to the office with Nike."
Hint 4
Don't go looking for running shoes; Nike is the Greek goddess of victory, and this is a Greek art museum, after all.
Hint 5
Go to the group of white marble statues, and look for one of a headless woman with wings -- you may recognize it, in fact, since it's a famous image, and in case you don't, it's labeled, "Statue of Nike -- Winged Victory." The key is on the pedestal by Nike's right foot.