Mounting is when you want to mount a file system on a specific device. Like your game doom3. If you want to read the contents on the disk you have to mount it first. Then mount commands should be set up in your /etc/fstab. It should look something like this...
/dev/cdrom /mnt/cdrom iso9660 users,noauto,owner,ro 0 0
If you want to mount your cd, just run the command...
To unmount it...
You can find more information with 'man mount' from the terminal.
You can get the script for doom3 here
All you need to do is copy the pak* files from your discs to /usr/local/games/doom3/base after you executed the script.