I am a current visitor visiting the USA. I do have a Visa for the USA. Can this also be used to enter Canada?
No. You must apply for a separate Visa to enter Canada. You may get information on visas to enter Canada, by contacting Citizenship and Immigration Canada. Their web site has information on what you will need, and how to apply for a visa. The web address for this is: http://www.cic.gc.ca/english/visit/index.asp