A quick look into the menu or help file will give you the answer, you are looking for DVD-ROM.
One thing that is not well defined in the help is that your files need to be in a double folder structure. Name the outer folder whatever you what, then inside that you make an folder normally called EXTRAS or DVD_ROM (but it can be whatever you want), Put all your files in this folder. Then within DVDSP you point the DVD-ROM to your outer folder. When you burn the bisc within DVDSP your inner folder and files will be included on the disc.