what type of video file are you starting with? do you want to type you own subtitles or add a subtitle file that exists already? Have you check the Afterdawn guides? you can also check out this mini guide on how to add subtitles to DVD http://www.vso-software.fr/faq/add_subtitles_to_dvd/add_subtitles_to_dvd.php
You can not add subs to an existing dvd movie. Rip the movie, and add subs with this program: ConvertXtoDVD [new version 3] google that, I don't have directlink.
Subtitles can be added to an existing DVD (a DVD that has been ripped to the hard drive)- but the movie files have to be demuxed to elementary streams (separate audio and video) - then the new subs are muxed together with the audio and video to create a new movie. It's relatively easy if you don't mind losing the menu and chapters - It's more complex if you wish to keep the menu intact. The subtitles have to be in .sup format. 'Rejig' can demux/mux the ripped DVD. http://www.afterdawn.com/software/video_software/video_tools/rejig.cfm