I have installed Slackware 9.0 on VirtualBox and I've never used it (slackware) before. How do I access a CD/DVD inserted in my laptop through the KDE?

Probably you will have to mount it manually. As a root
Actually /dev/cdrom is a link to a device: it should be /dev/hdb (as Slackware 9.0 works only with IDE/ATAPI drives). But notice if you have a relatively new-brand there may be problems with mounting due to outdated drivers. Out of box common user has no rights to mount (or umount) devices. However you can add an entry in /etc/fstab file
I am not quite sure the above is valid. Read SLACKWARE-Howto form a root catalog of the CD-install (and all other READMES from that catalog). Read also a mail to the root, after the first login.

May I ask why you installed 9.0? The current version is 13.37? I think support for the Virtualbox specific hardware is much better with a recent version.

Well, I have this very old program which I found a lot of difficulty in compiling on modern linux versions. So when I asked a person who had worked with this program, he had suggested that I go for Slackware 9.0. My main problem here is that I have a few files on my host OS which I want to transfer to the guest OS(slackware). So I thought may be I could copy these files to a CD and then copy these files into a folder on slackware. Is there an easier alternative to this that you know?

Set up an FTP server or a Samba share on the Slackware install. You can copy your files through the virtual network then.

KDE is using HAL to mount devices. But there is no HAL (am I wrong?) in Slackware 9.0. So try my solution.
You can copy all files as a root and next change an owner and a group for that files. I was running Slackware 9.1 and as I remeber it is simplest way. KDE itself cannot mount a CD-ROM except there is a proper entry in /etc/fstab. You can put on a desktop an icon for a device (right clck-> configure desktop) choose Behavior and a tab device icons - pick cdrom. Next click this icon with RB and edit "mount point" entry : this must be exactly the same as in the /etc/fstab and you should have privileges (as a user) to mount device. Right click on the icon give you mount/umount options. Note also an "eject" command. It does two things: umounts and opens CD-ROM ("eject -t" closes)