I have hit a point where my 6 mo old puppy's aggression has just become too much. Took him to a behavior modification trainer and hopefully it works. Have any of you had any experience with behavior mod, what were those exercises and did they work?

No experience but just a heads up my lecturer was a qualified behaviourist and she warned - make sure they're accreditted/have a degree in animal behaviour cause anyone can call themselves a behaviourist. Most are just trainers and don't understand the more deeper and complicated workings of a dogs mind...Cesar Millan being the obvious example.
What sort of situations does your pup show aggression? Just as a matter of interest.

Be VERY careful. If the trainer starts talking about needing to be the "alpha" and putting the puppy in its place, run! Using dominance-based techniques on an aggressive dog can make the problem MUCH worse.

He was talking about some alpha stuff but was saying he didn't want to challenge Walter because Walter has such a short fuse. That being said the guy hasn't gotten back to us in about a week so we may try to find someone else.

Walter is pretty food aggressive so you can't even take his bowl away if there is no food in it until he is done with it. Today I tried feeding him with the tug a jug and I was sitting on the stairs next to where he eats and he walked up to me and snapped at my hand. It ended up catching one of my fingers in his mouth which I think hurt him so he ended up going back after me and latched onto my hand. I don't like being aggressive back but I ended up having to grab him and pin him for a second so I could gain control of my hand back and hold him away from me. Also sometimes he would lay in a spot and then if you walk to near him in that spot he will get up and walk up to you and snap at you or nip at you. He also guards raw hides.

To top it off. Today someone brought a squeaky toy to the dog park which to me sounds idiotic. Walter ended up getting it for awhile after the owners dog dropped it. When the owner got it back he threw it for his dog, Walter chased too and the owner's dog snapped at Walter and a scuffle ensued. The owner got on one knee to separate the two (one in each hand) and Walter bit his hand.

Some of the issues are guarding some we think are just nipping real hard. Our vet, doggy daycare place, and the trainer have all said he is a very dominant and confident dog and that could be the reason but we just don't know and have hit a wall.

There is a lot of information on this site about resource guarding, remember resource guarding is a fear-based response, not dominant and far from confident. It can't be handled the way most people think it should be and being "aggressive" back will make his problems so much worse. I had to leave Snoopy, who also resource guards, with my mom for about a month and I'm still trying to repair the damage that was done. The biggest thing is trust building exercises.

I would have left the dog park when you saw an object you knew he would guard. Most people are sue happy and I have no doubt if that happened to me there would be a lawsuit on hand with the potential to end my dog's life. Prevention of the incidents is the first step to helping him get better.

Most people see an aggressive dog and they automatically assume that dog is confident, when in reality they are far from it. Most aggression can be chalked up to fear.

I'm sure a vet would be able to tell confident aggression from fear aggression. Sounds like he's overly confident to me...if you haven't already you can look up canine body language and it can give you a bit of an idea if he's fearful or confident rather than relying on someone to tell you whats what.
The trainer was right in saying he doesn't want to compete w/ your dog but alpha roles don't apply in the dog world which is why a 'behaviourist' who believes in the dominant/alpha stuff either isn't a qualified behaviourist or didn't listen during however many years of study it takes to become one.
I haven't done much research on it but I don't think resource guarding is always fear.... Achilles still guards his bones from our poodle...I'm sure my 26kg 1yo Husky isn't afraid of a 8kg 13yo Toy Poodle or sees him as any sort of threat, he's just not willing to share and will protect it cause he's bigger and he knows it. From memory the way to work around it is to convince the dog he doesn't have to guard his food because you'll give it back and there will always be more.

Resource guarding is the result of fear of losing an object, or losing control of an object. Trust is key with this type of problem. They don't necessarily have to be afraid of what their guarding from all the time, but for whatever reason when they have something valuable they are afraid of losing it to someone or something they wouldn't normally act out against.

A "leave it" or "off" (backing away from an object) command is very helpful with this type of behavior modification. I would suggest building up on these commands on low value items first, and slowly moving up the scale of what he sees as valuable.

As far as vets go, though I'm not sure if this is a regional thing or not, but vets are trained to treat physical ailments, not behavioral problems. It's kind of similar how most vets don't know anything about species appropriate diets either, if that comparison makes sense?

bgjacman, did you ever hand feed Walter?