What kind of visa I should apply for a visit to mainland china when visiting Hong Kong.

I am proceeding to Hong kong for Canadian Immigration Interview with family (Wife + 3 Children) to be held on 09 May 2013. I have already applied for visa of Hong Kong through Cathy Pacific off at Metropole Hotel Karachi. Taking this opportunity I also wish to travel by train to main land China for a short visit. What kind of visa I should apply for and how?