I'm assuming you're on informal misdemeanor probation. Typically you are ordered to do things by a certain date or the court schedules review hearings to verify that you are in compliance. If you weren't given a date or there are no scheduled review dates the court may never find out.
Regardless, I would recommend that you enroll in the AA meetings ASAP. If you are found in violation of probation the judge can sentence you to jail, up to 6 months for a first offense DUI.
Check the court documents that either (1) your attorney should have provided you or (2) you would have obtained from the court clerk if you were not represented by an attorney. Look for the minute order - that document tells you when you are required to register for and complete classes.
As suggested above, register ASAP. If your compliance hearing is approaching, it's better to show up having recently registered and (hopefully) attended one or two classes than to show up completely empty-handed.
I wonder if any of the attorneys above actually have experience with clients who were court-ordered to attend AA meetings.
There is no such thing as "enrollment" in Alcoholics Anonymous. You can usually find a schedule of local meetings on the Internet, or obtain one from the court or probation department. In many communities, the meetings are also listed in the community events section of the newspaper, or there may be an office phone number.
You can obtain proof that you attended the meeting by having the person in charge, usually called the "secretary," sign a form you obtained from the court. If you don't have a court form, take a small notebook or even a piece of paper... but whatever you use, don't lose it!
NEVER refer to AA meetings as "classes." Judges who deal with DUI or drug cases know that Alcoholics Anonymous, Narcotics Anonymous and other 12-step self help groups never refer to their meetings as "classes." (The judge may even be a member of one of the groups.) If you use the wrong term, the judge may think you aren't paying attention.
Some criminal defendants have challenged court-ordered attendance at 12-step meetings on First Amendment grounds because the programs have a religious component. However, if you find court-ordered AA meetings conflict with your religious beliefs, you should raise that objection as soon as possible after you become aware of the conflict, instead of waiting for a review and showing up to court empty-handed.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.