Yeah, sadly we can't use calculators 🙁. But you have to remember that the MCAT is not testing you on your mathematical skills.
Even though some problems may seem like you need a calculator, you really don't -- they expect you to use estimations and understand proportions.
I suggest to brush up on exponents, logarithms, decimals (and even basic arithmetic). Also, the Kaplan books do a decent job in teaching quick techniques to solve the latter problems. 😉