Every county is different and has their own rules and procedures. In some counties, you would be allowed to go forward without mediation. Obviously, in your county, they are telling you that you still must do mediation. Your best bet is to consult with an attorney in your area to find out how things work and what your options are. Even if you go forward, her lack of cooperation will most likely reflect badly on her and well on you in terms of a final decision so as frustrating as this might be, it may help you in the end for your custody case. Good luck to you.
I am assuming you are proceeding ProSe. If you are, then mediation is your best bet to get an agreement in writing and get this all behind you. I would ask for another appointment. If you don't get an agreement you can live with then you can always walk away.