When signing up, select the disabled access option on the 'Your Details' step of the join process. We'll then give you the details of your Gym Manager so you can contact them and arrange for them to meet you in the gym. They'll be able to talk through your needs and guide you through using the facilities.
Existing members can also organise disabled access through their gym manager. You can contact them using the 'Email this gym' button on the 'Overview' page of your gym's homepage.