You're doing nothing wrong. The disabled access tickets aren't shown or bookable online!
With a disabled wife (scooter user) I tried phoning the club, and Wembley's disabled access number, without success after several hours. I also emailed the club and Gemma rang back and sorted us out - tickets for self + wife in a disabled row (row 28 from memory) and 4 tickets for the rest of our group nearby.
In short my advice is email the club explaining exactly what you want, give your phone number, and you'll get a call back. They're operating under enormous pressure, not helped by the Ticketmaster page asserting that all the disabled spaces are sold, when they're not.
DM me if I can be of any further help