You should be able to manipulate parts on screen, but you need the right tool selected (and your graphics ard might stop you, too). The first time you load D|S it has the selection tool active, which doesn't allow any manipulation - there are also rotate, translate ans cale tools (of which the first is most useful for posing figures) and the Power Pose tool (sort of IK, with the option to pin different body parts). D|S remembers the last tool used when you relaunch it.