Your CD drive can be switched manually from host to guest and vice versa
using the Virtualbox system tray on the bottom right of the status bar.
Right click on the drive icon to choose that drive. You can also enable
Passthrough in the settings for the Settings for drives for the VM (before
you boot Windows). If you do this then you can use the drive in both host
and guest. If you are in Windows when you are using the CD drive, Windows
takes over. If you are in Linux then Linux takes over. To release the CD for
use in the other OS you need to eject it from the OS that has control so
that the other OS can use it.