I had this exact same problem. Which version of VirtualBox, specifically? I had 5.0.14 along with Ubuntu 15.10. Whenever I tried to install the guest additions things, there was a cryptic message saying,
The headers for the current running kernel were not found. If the following
module compilation fails then this could be the reason.
However, nothing failed to build, at least, no error messages appeared on screen to that effect. Additionally, the display driver did seem to work, and I was able to set higher resolution for the display, although it did not have the resolution of my native monitor.
Looking at the changelogs for VirtualBox, there was a mention in v.16 or .18 about fixing something related to SharedFolders or something not building correctly from the additions.
The "simple" solution I tried (note this 3 months later), was to upgrade both VirtualBox (5.0.20) and Ubuntu (16.10).
Then deleted all the virtual machines, recreated, reinstalled Ubuntu.
Then installed the guest additions stuff, either from ISO in the
C:\Program Files\VirtualBox folder (or wherever) or the
Then all I did then was to define a shared folder in VirtualBox, and choose to mount it automatically. And it was mounted under
* is whatever name you gave the share in VirtualBox.
The final task was to add my user to the vboxsf group, and logout and/or restart.
sudo usermod -a -G vboxsf <username>
A reboot and I was able to change display dimensions and read/write access to shared folder.