Go through the double doors on your left as you enter, or just follow a group of tourists nearby. If you're bringing the W2000, you need to buy the briefcase padding upgrade to be able to place it in the x-ray machine. Sneak into the staff room and get museum staff clothes from the man showering. Alternatively, it is possible to enter the toilet before the metal detectors and lure a Marine in there to steal his uniform, but you need to do this quietly or the mission might end before it begins. The easiest way to do this is to throw an object or the coin onto a hard surface, causing him to come to take it to lost property - this method can be difficult but nets you a marine disguise early on with little chance of discovery and makes suit retrieval easy. (and you can gain access to many area