You must download an .srt file. A good Website for all languages is:
Subtitles - download DivX subtitles from the biggest open subtitles database
Make sure your .avi file and .srt file have
exactly the same title (if not, rename them with a right mouse click).
Double click on the .avi file and BSPlayer will start the film with the subs.
NOTE: always check the uploader of the subs. This should be the same one as the movie upoloader. If not you maY have synchro problems.