Boot media, your CD perhaps is not readable, you can check if it can be opened on the Linux host side: seeing if it is readable there.
Virtualbox (vbox), you need to be in the vbox user group, will be there if Virtualbox installs correctly
--the simple way of getting usb for a plug in device, such as a data card (usb pendrive etc.), is to open that usb device on the linux host side, and then it will be available in the virtualbox guest
And since you would always be starting Linux before loading the Virtualbox guest, it is only a matter of a few clicks
--but if you don't want to do that and instead prefer it to be always available in the gust, you can edit /etc/fstab (Linux host) to include the usb device, or another way, a usb.conf file. There will be guides here these forums, use a search " lmde enable usb virtualbox" or something similar or use the Internet for like instructions/guides..