If you are ineligible to enter the United States, you may apply for a waiver of ineligibility. This will involve completing an Application for Advance Permission to Enter as a Non-Immigrant (Form I-192). This form is available from DHS/CBP from CBP at either a U.S Port of Entry, or at any Canadian airport with a U.S. preclearance facility (Vancouver, Calgary, Edmonton, Winnipeg, Toronto, Ottawa, and Montréal-Trudeau). The form(s) can also be downloaded from the Internet at the USCIS web site. *CBP Ottawa Airport accepts applications 11:00 am to 3:00 pm, Monday to Friday.*
Right from their website.