Can't install Guest Additions using VirtualBox, Ubuntu guest OS, Win7 host OS

As I mentioned in the comments, seems the ISO is not mounted. Here's what you can do

• Ensure that the guest additions ISO is available to the host. You can do this by clicking on Devices menu -> CD/DVD devices and point to the GuestAdditions ISO file. The Guest additions is available in Program Files\Oracle\VirtualBox folder

• With that mounted, type in the below

  cd /media
  mkdir vbox
  sudo mount /dev/sr0 vbox/
  cd vbox/
  

• Now, the guest installation ISO is mounted as /media/vbox

• Run the Guest additions installer by tying ./VBoxLinuxAdditions.run