Even if a truly random element exists in the makeup of the human brain, it doesn't guarantee 'free will'. It just means you wouldn't be able to predict what happens next.
Let's say the universe was completely deterministic and that you knew the motion and location of every particle and force in the universe. You use this remarkable ability to predict, with success, that I will say the word 'platypus' at 8:58 pm by extrapolating how the motion of these particles and these interactions will follow, right down to how the neurons will fire in my brain.
Now, let's say the universe was not completely deterministic, and random subatomic motions of particles (or 'probability fields' or something) cause some of the neurons in my head to fire in a way you can't predict. You're knocked off your feet with surprise when I open my mouth and say 'anteater' instead.
Does this mean I have free will? No, it simply means that the operation of my brain, and the firing of my neurons, were influenced in part by random factors instead of preceding causes.
Let's compare to video games. I'm a fan of playing first-person shooters, like Half-Life, where you engage in firefights with bad guys. Let's say you're a programmer designing how the computer-generated villains act when they attack you. It would be a very poor game if the villain took two steps to the right, fired three shots at you, and took three steps to the left, and shot at you four times... every single time you encountered them.
So, you cook up an algorithm to plug in some random behavior, using random data variables as a seed - the temperature of the video card's CPU, the time in milliseconds, whatever.
Success! You, as a player, cannot reliably predict the behavior of the enemies! The game is fun again!
However, you wouldn't, not for a second, consider those computer-generated baddies to possesses 'free will'.