You must show him what you want and keep repeating it.
Everytime you go outside, take his NOSE and ring the bell, give your command word (outside potty - or whatever you want to use), open the door, and go outside.
At first, when he learns to ring the bell, he will ring it all the time for a play toy. You MUST open the door every time he rings the bell though. LOL After a few days of *playing* constantly with the bell, he will stop playing with it.
Mine were taught to ring the bell with their noses, as I didn't want them using their paws. Make sure the bell is hanging low enough for your puppy to reach with his nose. You can raise the bell as your puppy grows.
Good luck with your puppy!!