Subtitles must be turned on to see them on the DVD.
You can use the remote control for your DVD player to turn subtitles on and off. You can do that now to see whether you actually succeeded in including them on your DVD. If you did, users can use the remote control and you are all done.
If you want them to appear when a user starts the disk, you can set them on by default. See "Set the Default ... Subtitle Track here:
A user can turn them off with their remote, or yoiu can create menus for this. See "Set buttons to select audio or subtitle tracks" on the same link above.