It's different if you want to getyour dog to reatrieve for fun, or if you want to get him to retrieve like in obedience.
My dog was never a big retriver, she doesn't like to play, but:
Even if it sounds odd, it's important waht kind of a ball you use. Pami is only willing to play with one type of a ball, or only the ball of a specific dog we know.
If you want to get your dog to discover that retrieving is fun, there is a way I know you can try (I wasn't patient enough to try it properly...
Take a ball in your hand, and play with yourself. Make sure the dog sees how you enjoy playing with it. If he tries to take a closer look at the thing you play with, go a littlebit further, don't let him get it. This will make the misterious thing even more intresting for the dog.Then, if you see that the dog is really enthusiastic, and really intrested, just let the ball roll to the dog. Let him get it, then take it from him gently. Make sure thatyou show how happy you are that he got it. Then start from the beginning. If you are patient enough, you will play fetch with your dog soon. You can do this with a tug toy too.
I hope I helped, and I wasn't too confusing. If I was, feel free to ask...