Just drag the subs to the CTD menu after you drag the title. Should work just the same with a plus as youdrag the subs to the title. Make sure when you watch the video you go into your DVD settings and apply the SUBS.
As long as you have the correct subs in the same folder as your movie convertx adds them automatically,no need to drag the dugs or add anything just drag the movie and the subs automatically add themselves keep it simple!