I had some trouble installing VirtualBox Guest Additions on Ubuntu server on my Mac. I had to do the following:
cp /Applications/VirtualBox.app/Contents/MacOS/VBoxGuestAdditions.iso ~
Attach the iso file above to VirtualBox.
As root on the Ubuntu system:
mkdir /mnt/cdrom mount /dev/sr0 /mnt/cdrom apt-get install gcc /mnt/cdrom/VBoxLinuxAdditions.run
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