I am 69 years old. Although retired, I teach fitness classes, and my employer does not withdraw anything. How can I use the CPT 30 form if my employer doesn't want it?

Unfortunately, your employer is not taking deductions because he does not want to remit the other half of your CPP.  There isn't much to be done about him if you want to keep your job.  What he is doing is illegal, but you would have to turn him in to CRA which would not be conducive to a happy working relationship.