First of all, that InterVideo configuration window is legacy and doesn't actually do something. So, forget about it.
Second, does your speaker configuration is set to 5.1 in audio configuration (Window's configuration, not BSPlayer's) ? Usually, by default, on a laptop this is set to stereo.
When you play 5.1 sound, in AC3Filter's main window can you see input signal as 5.1 or is it stereo ?
Output is set to 5.1 ?