Can't enter DVD drive as source in DVDFAB express

I know this question is in one of the forums but I was unable to locate it so I am sorry to repeat it.
I am trying out DVDFab Plat. I used the Express part of the program yesterday and it was fine. Today, when identifying the Source, it would not accept my DVD drive as a source and replied that “the folder is not a DVD folder” and that “file (D:\Video_TS\Video.IFO) doesn’t exist”. What should I do?