Depends what type of chain tensioner you are talking about. I'm assuming your talking about the kind that look like a mini derailleur. Those type should just mount to the spot where a regular derailleur would mount.
Or if you have one like in the picture below, it would mount straight on your axle. Unsecure image, only https images allowed: http://www.jensonusa.com/store/imgWrapper.aspx?img=/ch/CH309Z10BLK__14.jpg&type=2