You need to communicate with the court, not the District Attorney. Understand however, that the court has discretion, and will not grant the Marsden motion simply because you and your Public Defender do not get along.
Be prepared to cite very specific details of inadequacies. Practically speaking, you might ask to speak with your Public Defender's supervisor.
If you cannot calendar a Marsden hearing before your trial readiness conference, let the Judge know of your concerns during the conference. The Judge may immediately clear the courtroom and conduct the hearing.
The judge will not read your letter; however, if your attorney refuses to inform the court that you wish to make a Marsden motion, speak up and say you want to make the motion or if you stumble say you want to fire your attorney. At that point the judge will clear the courtroom of everyone but courtroom staff, you and your attorney and you can explain your problem. That you dislike each other is probably not enough. You will have to show the court how your attorney's defense has prejudiced your case.