1. Make sure that the movie and the sub files (.idx and .sub or .srt) are in one and the same folder.
2. The video and the subs should have the same file name.
If you use VLC Player and the files are in the same folder with same file names, VLC can automatically load the subs.
If you use VLC and the files have different file name, you can still load the subs manually by doing the following:
– click on VIDEO tab -> choose SUBTITLE TRACK from the dropdown menu.