You should be able to morph the mouth in the objects properties window. If you are using one of the Daz people or the people that came with the program, in Poser5 - select the head and left click on menu item Object then Properties. If the character you are using has the morphs for mouth you can adjust them here. In poser 4 you should see on your right a bunch of dials. I usually start with mouth open or mouth O dials and then Smile dial for early models. If you do not see these controlling dials for character then the character wasnt designed for that and you will have to go to a 3d modeling program to make changes - would be easier to switch characters ;-)