My Owen is in agility - we started when he was about a year old. We started becuase he was a nervous dog - agility has built his confidence like you wouldn't believe!
Anyway - Owen had a HUGE fear of the tunnel too. Have you tried using a clicker (or any other marker)? I personally use a clicker and with Owen it has really worked wonderful.
Since Owen was a such a nervous dog I had to start from the BASICS. I would click and treat (CT) when he would just look at the tunnel. Then I would CT when he would get close to the tunnel, then closer, then sniff it, then put his nose in it, then his head, his paw,... you get the picture.
By taking your time and rewarding for ALL the small steps you are building condifence in your puppy and getting them to enjoy the tunnel.
Now Owen has NO problem with the tunnel at any length and in any shape.
DO NOT push or pull (force) your dog onto or into any equipment. Take your time and reward your dog for their successes - no matter how small they may seem.
Have Fun!