Has the dog been exposed to other cats before? If so how has he acted? If his is good around cats, all the easier but if your cats are freaked out by him then it might take a little work.
First don't expect them to be best friends. I have 2 dogs and 4 cats and they hardly ever interact. They all get along well but they just don't find the other interesting. Except Xavier who loves the dogs and loves to rub on them.
Anyway when they meet don't have the dog go to them. Have him lie down and relax, maybe give him a toy to play with. If the cats can see he's not a big threat then they won't be as scared. If they've never seen a dog before this may take a loooong time.
If the cats will willingly go up to the dog and check him out make sure he stays calm. If he gets excited and scares them then this will be a big set back. I guess the way you want to approach it is let them interact in their own time. If you stick the cat in front of the dog right away, the cat is going to get scared and run and not want anything to do with him.
If this doesn't work then I'm sure the other members will have some good suggestions too.