How is it that an omniscient being can predict what choices you make? That could only be true if choices are part of the causal chain, just like everything else. If you think of billiard balls bouncing around, hitting each other and the table sides in predictable ways, that is an analogy for cause and effect. If a choice is just another predictable billiard ball, then that is incompatible with free will.
I'm not saying I believe in predestination, in fact I don't, but here's the theory behind it:
God has decided before you were born whether or not you will go to heaven or hell. Based on where you will go, you make decisions, unconsciously, to get you there. For instance, if you will go to heaven, your actions will end up displaying that through good works and such. If you are predestined to go to hell, your decisions will end up placing you there. Basically, an illusion of free will, but God decides beforehand and therefore you can't change your fate. Sure, maybe you start off murdering somebody, but then start doing good and end up going to heaven, but all that was predetermined. God had predestined you to go to heaven, so even if you started off "evil," eventually you would have done what was required to go to heaven.
In effect, you get the illusion of free will, but there really isn't any.
At least, I think that's how it works. I don't believe in it, so I'm no expert and not completely sure.