If you already downloaded a
VBoxGuestAdditions.iso file to your host filesystem, you should be able to install the image from the VM's
Devices menu using
Insert Guest Additions CD Image...
Note that does not actually install the guest addition components, it just makes the ISO available as a virtual CD. On Ubuntu Server guest OSes, it does not even mount the virtual CD within the guest system.
Alternatively, you can download the ISO straight into the guest system from the repository, using
sudo apt-get update
sudo apt-get install virtualbox-guest-additions-iso
Once you have made the ISO available by one of the above methods, you must mount it somewhere in the filesystem. I am going to assume you want to use mountpoint
/mnt for that, but you can create a different mountpoint if you wish.
If you inserted the ISO image as a virtual CD:
sudo mount /dev/cdrom /mnt
If you installed the guest additions ISO package from inside Ubuntu:
sudo mount /usr/share/virtualbox/VBoxGuestAdditions.iso /mnt
Finally, navigate to the mount point and run the installer: