You probably have no "Intervideo AC3" installed.
I would advise to install AC3 filter. You can find a direct link where to download AC3 filter in
adicoto's list of codecs.
After installation you can access AC3 filter's settings from BSPlayer (with videofile loaded) through
rightclick > Options > Filters > Advanced
(for general setting purposes you can also access it directly through
[Start] > Program files > AC3Filter > AC3Filter Config)