Also just so you know, you can't have a watch or anything, and if the proctors accidently let you start the test without realizing it, and the see it, they can void the test, etc.
If you read the MCAT essentials you will see that the only things you can bring into the exam room are pencils which they provide you with, scratch paper which they provide, and a pair of ear plugs which must be in a sealed bag. You can however bring snacks and drinks and meds that you put in your locker and can access during breaks. Also, there's no reason why you should need a stop watch because of the presence of a timer on the computer screen. Have you been taking the AAMC practice tests? They do an excellent job at recreating the real deal.