If you set the Cd drive to ISO to do the instalaltion then you have to
change it back. You also should either change it to passthrough so that it
can be shared (settings). Otherwise it can be dedicated to the host machine
or the client, but not both. Finally you can switch drives in the system
tray of the VM Windows by right clicking on the icon for the CD drive. If
you are using a usb drive then it won't work unless you are using PUEL from
Virtualbox.org and not the OSE from the repositories.

Finally you can use a Shared folder on the host machine to move files back
and forth or use drag and drop if you have that enabled.