Adding file to iso file using nero 7.0

Hi i have an iso data dvd file which i burned already. Im trying to add some files but when i try to create a new continue multisession dvd the dvd shows up as volume with out any of the files. it looks like a music dvd. and when i add the files and burn it, the files show as another volume. and when browse the dvd the files i added are not there. Any suggestions. Thanks