Try to register at the link below I have suggested this link to others and they have had some success. You will of course have to provide some info such as your father's birth name and full date of birth. Its possible they are looking for info on him as well but unless you post his date of birth and adopted name you might not get very far. The link below provides tips on how to search including searching the newspapers. Sometimes people put in a search notice in the paper when the adoptive child turns 18. Also you might want to search old telephone directories that will provide addresses.
Since she was a nurse you might want to try searching the Nursing Alumni they keep really good records on nurses which include their maiden names, I work for a nursing home and we get flowers delivered all the time for residents that are nurses because the Alumni found them. You could try posting on a Nursing Alumni message board, you don't need to say what you are looking for her for but maybe post her name and that you are looking to contact her and leave your contact info. They might even forward the letter to her for you. Worth a try.http://www.canadianadopteesregistry.org/
Try posting at cousin connect I've attached the link below they have an area for people looking for for adoptive families. http://www.cousinconnect.com/