Search for threads discussing subtitles - there's a lot of 'em in here.
Bottom line, HB cannot read the subs from the source material, so you have to pull the subtitle (*.srt) file from the DVD or find it on the 'net (there are plenty of sources), and add it to your clip by going to the subtitle tab and clicking the add External srt option.
However, HB will only give you a soft subtitle, like closed captioning, so you have to turn on subtitles when watching the movie. Also, unless you have a "forced only" or "non-English only" srt file, it will have subs for all dialog. You can sometimes find non-English only srt files on the 'net, and people more clever than I can pull the non-English only file from the DVD. Otherwise, you have to edit the file to take out the English dialog subs so that only the non-English remains (this can be done using Jubler - which is free).
If you want to get very clever, there are ways to burn in subs to the picture, so you do not have to turn on the subtitles to see them. There is an explanation of how to do that in this forum, but it does require the forced only subtitle track as a starting point.