My understanding is that Canada treats DUI as a felony and as such will deny admission to those foreign nationals attempting to enter Canada that have these on their records. The bad news for US citizens is that our government shares the NCIC criminal history database with several governments for immigration/criminal justice purposes and Canada is one of those governments. You may be able to enter if you make special arrangements ahead of time. Try contacting the Canadian Embassy or your local Canadian Consulate.
Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.