Open up VBox then: Devices > Install Guest Additions
This will mount the Additions iso. It should run automatically but if it doesn't it should be under /media/VBOXADDITIONS.....
sudo sh VBoxLinuxAdditions.run
Once you install it you no longer have to do any manual mount it's automatically mounted at:
You may get an error because you are not a member of the vboxsf group so you will need to add yourself:
Then logout and login again.
sudo gpasswd -a username vboxsf