Welcome to the forums patty304.
Are you trying to compress a dvd movie on the hard drive, or is this an original dvd disk in your optical drive?
If it is an original disk, you should know that DVDShrink hasn't been updated to deal with newer protection schemes since 2004 or so. You need to use more modern decryption programs with newer movies. DVDFab HD Decrypter, DVDFab Passkey or AnyDVD come to mind.
If it is a dvd video that has already been ripped to the hard drive, you might want to run it through FixVTS before trying to open it with DVDShrink. http://www.videohelp.com/tools/FixVTS