Press whichever function key is associated with Kernel Option. I can't quite make it out, but I believe it's F6 in your case. You will then get the screen AS was showing. Then proceed with the rest of the instructions.
You might as well go ahead and login as user root. You'll need to be root to create a mount point for the CD, mount the CD, then run the guest additions reinstallation. After logging in, when you click on Devices > Install Guest Additions in the VirtualBox window, the CD will be available to the virtual machine, but it won't automount. So, to mount the CD manually and reinstall the guest additions, run as root:
mount /dev/sr0 /mnt/vbox
First line is mkdir [space] /mnt/vbox.
Second line is mount [space] /dev/sr0 (that's sr-zero) [space] /mnt/vbox.
After reinstallation has finished, issue either reboot or halt command.