mandrake
 

I try to mount the cdrom from the command by entering.
mount -t iso9660 /dev/scd0 /mnt

this is the error message I receive:
mount: block device /dev/cdrom is write-protected, mounting
read-only

by the way, how can I install kde3 from a dvd media from the command:

please help:

This is not an error message it is just a friendly warning, telling you that you cannot write to cdrom (even if you could how'd you expect it to without use of cdr-tools?), so you should be able to access your cdrom contents by going into /mnt directory
cd /mnt
ls -l|less
as for your second question - mount DVD media the way you did it before don't pay too much attention to the warning about mounting read-only, cd into /mnt directory then find your way to KDE 3 RPM's become root with su command and issue
rpm -Uhv kdebase*.rpm kdelibs*.rpm
and then install everything else KDE related using the rpm command with -Uhv option - it tells to update the package if the previous version exists otherwise install the package if there is no previous version.
Good luck

Or, since you're using Mandrake... mount the DVD, run rpmdrake and 'define a new source'. Add the directory with the relevant Mandy RPM files, and let it do its thing. If you install the RPMs manually, you'll probably find a load of dependencies that are self-satisfying (i.e. RPM file X requires a lib from RPM file Y, which requires a lib from RPM file X, etc), so letting rpmdrake sort this out for you is potentially an easier option. BTW, there should be docs on the DVD about how to update automagically (look for something called urpmi).

Hope this helps.