You can create the subs alongside a preview of the video file using this. (DVD Lab Pro doesn't let you do this easily and even admits this in the user guide).
Once you have the subtitles completed, you can import the srt file into DVD Lab Pro (or other DVD Authoring tools).
DVD Lab Pro lets you change the position, look, font etc of the subtitles. You have to compile the subtitles, which then creates bitmaps that display the subtitles. When compiling the dvd, it adds the compiled subtitles.