The ADA does not apply. The Air Carrier Access Act applies to commercial airlines in the US.
Under the ACAA, airlines must offer pre-boarding to passengers with a disability who identify themselves at the gate as needing additional time or assistance to board, to stow accessibility equipment, or to be seated.
If you feel your rights under the ACAA have been violated and you are still on the aircraft/at the airport, request to meet with the airline's CRO.
If you would like the DOT to investigate your complaint, submit it to:
Aviation Consumer Protection Division
Attn: C-75-D
U.S. Department of Transportation
1200 New Jersey Ave, SE
Washington, D.C. 20590
You can submit complaints via the DOT's website here:
http://airconsumer.ost.dot.gov/escomplaint/es.cfm
or on paper here:
http://airconsumer.ost.dot.gov/forms/382form.pdf