How to get dog to be nice to cat?

I had to move in with my fiance and his parents because my mother had to downsize and could no longer afford to have a place big enough for everyone to live in. Anyway, my fiance's parent's have a 100 lb lab...with no brains. I swear to you, he is the stupidest dog I have ever met. Now my cat took about 5 days to start to adventure around and I even took her downstairs with me. Then their crazy dog comes up to her and they just sniff each other...I'm like "ok, this is good." My cat wasn't even afraid or anything. But, she starts to move, not anything quick, just moving and the dog starts going berserk and chasing her and slapping both paws on the ground in front of her. So of course she finds the closest spot to hide and growl from. Does their dog think cats are his personal toy or something? I have seen him act this way around their cat. How can I make this dog behave? They have no problem with me disciplining their dog...they actually think it's good. So that's not an issue. But I just feel kind of hopeless because whenever this dog sees a cat he completely losses it as if he couldn't get any stupider! I don't know what to do...