For me (and I'm kinda new to Trisquel) I made sure I had the following
installed:
$ dpkg -l | grep virtualbox
ii virtualbox-guest-dkms
4.1.12-dfsg-2ubuntu0.5+6.0trisquel1 x86 virtualization solution -
guest addition module source for dkms
ii virtualbox-guest-utils
4.1.12-dfsg-2ubuntu0.5+6.0trisquel1 x86 virtualization solution -
non-X11 guest utilities
ii virtualbox-guest-x11
4.1.12-dfsg-2ubuntu0.5+6.0trisquel1 x86 virtualization solution - X11
guest utilities
You should be able to install those with:
$ sudo apt-get install virtualbox-guest-dkms virtualbox-guest-utils
virtualbox-guest-x11
As I recall (and my memory *is* horrible) things just worked after that.
In this case I'm using Trisquel 6.0.1 inside VirtualBox 4.3.10 under Windows
8.1 (don't judge please ;)
