The thing about this is, first of all God is beyond our full understanding, so any explanation will fall short. God is not bound by time, right? He created time/space/matter, so by very definition cannot not be bound by any of them the way we are. We do have a will, but honestly don't really see it as exactly a "free will", because whether we are in Adam or in Christ we are bound by our natures, but that aside we do have a choice, and just because God already knows that choice does not mean we didn’t make the choice.
I don’t find that a hard concept to grasp, but again I can’t possibly fully comprehend what “outside of time” truly means as I am bound by it. As far as free will goes, I once heard an illustration I really liked. Say you have a vulture in a room, you go in and set a bowl of raw meat and a bowl of carrots in front of him. You hold him back for a second and tell him how good those carrots are for him. You just let him know they’re really tasty, and these things are going to help your health and eyesight, they’re just so darn good. When you let him go 100 out of 100 times he’s going for the meat, because he is bound by his nature. Same test, but this time with a rabbit, and you tell him how beneficial eating meat is, and how it will improve brain function and increase his energy. 100 or of 100 times the rabbit, by nature, will go to the carrots. I think that we are bound by our natures in much the same way, yes we always have choice, but are wills are bound by our sin nature, or we are reborn and bound in Christ. Praise God.
That’s how I have come to see and understand it anyway, for what is worth.