yes I had the same problem when I first installed what you do is put in a cd not a dvd and then
make sure you mkdir /mnt/whatever first!
mount -t iso9660 /dev/hdxx /mnt/whatever
for some reason it wont work with dvd's at first untill you mount with a normal cdrom disk try to mount your slackware disk it works best
if that don't work try
mount -t udf,iso9660 /dev/hdxx /mnt/dvd
might work then edit your ect/fstabs to mount it automatic when you boot up
xine should read your dvds here is the video lan lib for css
http://developers.videolan.org/libdvdcss/
hope this helps a little I only got a few hours of sleep last night and I don't even know If I'm awake right now