The fast way is open it with the autorun prompt. The steps are 2.
- Install the VirtualBox Guest Additions from the VirtualBox menu. Now in the Ubuntu's Destop you will find the CD link, something like VBOXADDITIONS_XXXX
- Right click on the VBOXADDITIONS_XXXX icon on Desktop and select 'Open with Autorun Prompt'. Insert your password.
Wait the end of installation (about one minute) and restart your SO.
Have a nice day