That's often the problem in that most states do not accept completion of a program in a foreign state; in particular the DMV in CA won't accept it for your obligation with them. You may not have a record of completion but see if you can find the original provider and check with their records. Other than that, in your letter give the judge some personal insights you gained from the class and perhaps a letter from someone else, not related to you, who knows you participated and completed the program.
I agree with the other attorney's answer. In addition, I suggest contacting the alcohol school that you did go to and try to get some proof of your attendance and completion. Also, hiring an attorney to help with this would almost certainly make things much easier. Good luck.
You were given the wrong advice. You should never write a letter to a judge. You should retain a lawyer to provide documentation to DMV so that you can get your privileges re-instated or whatever it is that you are trying to accomplish. Please choose best answer if this helped you.