Kurt writes, "Many years ago when I was a permanent resident in Canada, I did not have a visa to the US but I showed up at the border to collect a car that I had purchased from an American seller. The agent at the US border let me accept the car and cleared the paperwork but handed me the form I160A (refused admission to the United States of America). Since then I have naturalized as a Canadian citizen and would like to visit the US. What should I do? Do I need any waivers? Do I need to file any paperwork?"
You can simply come as would any other Canadian. Just show your passport and you will most likely be allowed to enter. Because you arrived earlier without proper documents you were not allowed to enter but you were not barred from reentry because you broke no laws and were legally asking to enter. In fact you would not have needed to do any paperwork now even if you were not a Canadian citizen as long as you applied for a visa ahead of time. It does not mean that you will be definitely allowed entry because each and every entry of a non-US citizen (even with a valid visa) into the country depends on the discretion of the CBP agent (in your case, the record might show that you were refused entry because of lack of visa and the officer may see that now you are authorized to enter) and in case the officer concludes that since you tried to enter without proper documentation before, you are more likely to break other American laws, you maybe refused entry again. If that were to happen, you should hire an attorney (many of them have offices next to the border and help in such matters including accompanying you for crossings) and they can approach the agency on your behalf.
It does not mean that you will also be able to apply for other types of visas and get them. Short term visitor entries are routine and are approved by the border agent after making a judgment about your documents and intent. Other visa applications will specifically ask you if you were ever refused entry into the US and that might trigger an investigation into what happened at the border. Therefore, I suggest that you should work with an immigration lawyer to apply for a visa.