Where in the past I use to work with the additions CD-rom, I now use the package that Ubuntu offers called
virtualbox-guest-additions-iso.It is important to be aware about the fact that Ubuntu has split things up a bit and thus you have other packages to install too if you need some functionalities like vitualbox-guest-dkms, virtualbox-guest-x11 and virtualbox-guest-utils.
These virtualbox-guest packages are DKMS aware, which means that they can update without changing the whole kernel.
A common operation is to share a directory with the virtual machine. You do this by setting up a directory and permanently mounting it. It will be mounted in /media.