If you experience issues with installing the Virtualbox Guest Additions it could be that the build environment, dkms or kernel headers are not installed.
Messages typically look something like the following:
/tmp/vbox.0/Makefile.include.header:97: *** Error: unable to find the sources of your current Linux kernel. Specify KERN_DIR= and run Make again. Stop
Install the following packages and retry the Guest Additions install:
# apt-get install build-essential dkms linux-headers-generic