If you have any conviction in the United States, including a DUI, you may be refused entry to Canada. This information from the Canadian Government's Official Immigration Web Site, specifically mentions DUI as a reason visitors may be excluded:
"Criminal inadmissibility
If you have committed or been convicted of a criminal offence, you may not be allowed to enter Canada.
Criminal offences include both minor and serious offences, such as theft, assault, manslaughter, dangerous driving and driving while under the influence of drugs or alcohol. For a complete list of criminal offences in Canada, consult the Canadian Criminal Code.
If you were convicted of a crime when you were under the age of 18, you can probably still enter Canada."
In general, people are considered to be inadmissible to Canada due to past criminal activity if they were convicted of an offence in Canada or were convicted of an offence outside of Canada that is considered a crime in Canada. In order to enter Canada in the future, these people need to obtain a Temporary Resident Permit or Approval of Rehabilitation at a Canadian Consulate or Embassy.
You may be deemed rehabilitated if at least 10 years have passed since the completion of the sentence imposed for your crime, including any probation period. For example if you were convicted of driving under the influence it must be at least 10 years since your full driving privileges were restored and the probation period, if any, has ended.
