If mainbuttons.py is not there (or Poser can not find it), then there is something wrong with your installation. My first reaction would be to suggest to uninstall from C:Graphics the reinstall using the default installer settings. This will put it within C:Program Files. Other than that you may want to go to SM's customer support.

When Poser is installed it sets flags so that it knows what modules went where. It looks as though these flags have got lost so Poser can not find the Python library. If it were me I'd reinstall.