It sounds to me like you have a "sealed" sub, so I believe placement is a little easier because you don't have to worry about the "boomy" effect as much as you would with a ported design.
Usually one of the better places to put it would be near the front left / right speakers. If this is not an option, it has been suggested to set the sub right were you sit [chair, couch etc.] then play some heavy bass, get on your hands and knees and crawl around until you find a place it sounds the best and place the sub their.
Hope this helps.