I dont have dmks installed...
...and I just installed the Guest Additions like so:
root@bt:/media/dvd$ apt-cache search dkms
dkms - Dynamic Kernel Module Support Framework
root@bt:/media/dvd$ dpkg --list | grep dkms
(Actually an update of the Guest Additions, but same deal.)
Select "Install Guest Additions..." from the Devices menu on your BT4 VirtualBox window, then
root@bt:/media/dvd$ mount /dev/dvd3 /media/dvd
mount: block device /dev/hdc is write-protected, mounting read-only
Verifying archive integrity... All good.
Uncompressing VirtualBox 3.1.6 Guest Additions for Linux........
VirtualBox Guest Additions installer
Removing installed version of VirtualBox Guest Additions...
Building the VirtualBox Guest Additions kernel modules
Building the main Guest Additions module ...done.
Building the shared folder support module ...done.
Building the OpenGL support module ...done.
Doing non-kernel setup of the Guest Additions ...done.
You should restart your guest to make sure the new modules are actually used
Installing the Window System drivers
Installing X.Org Server 1.5 modules ...done.
Setting up the Window System to use the Guest Additions ...done.
You may need to restart the hal service and the Window System (or just restart
the guest system) to enable the Guest Additions.
Installing graphics libraries and desktop services components ...done.