We did what it sounds like you are doing. Everytime we let him out we would say, "Outside, Ring the Bells" and then ring them and praise him as he went out. For the first several days we just did it for him. Then we started pausing after saying, "Ring the Bells," to give him a chance to do it himself. If he didn't respond, we would ring them for him and praise him as he went out. After a bit, he would nudge the bells with his nose when we would say, "Ring the Bells," and not long after he started ringing them on his own. The kicker is that he only nudges them gently so it was easy to miss at first.
We have 6 people to the one dog living here so we just had to sit everyone down and tell them what to do. Even the 4 yr old got in on the action.
Now we have the problem that he wants to go out every 10 min sometimes!! We have gotten to the point where we will ignore him if he just came in a few minutes ago and hasn't eaten or taken a drink since coming in.
Good luck and keep with it. Oh - we didn't use treats because we were warned that it might teach them to ring it all the time to get a treat. I guess it works for some though! I know that some people out a little bit of peanut butter on the bells to encourage touching with the nose.