.iso is a disc image. You need to set up your Virtualbox virtual machine to mount the VBoxGuestAdditions.iso as a CD-ROM inside the virtual machine. Then it will appear in My Computer on Windows as a CD, and you can double-click on the installer in there to install the guest additions.
I try to treat the cause, not the symptom. I avoid the terminal in instructions, unless it's easier or necessary. My instructions will work within the Ubuntu system, instead of breaking or subverting it. Those are the three guarantees to the helpee.