I can't be definitive, but generally speaking, you walk up to any usher and ask them to sit in the wheelchair area, they will help you out, get a chair, the works.
Once a friend of mine came with a stroller and baby, and they let her and I sit in one of the handicapped areas.
I think each platform is somewhat first come, first served.
From their website:
Sixteen platforms, located throughout the Center, accommodate wheelchairs and folding chairs, allow patrons with mobility limitations and guests to sit together. Seating is limited to two tickets per order. Seats, located next to these platforms, are also available on a limited basis. Tickets for most events are available at the Bradley Center. If you would like this seating for an Admirals game, please call the Milwaukee Office for Persons with Disabilities at (414) 278-3930.
