First mount the Guestaddition iso on the new vm.  Start your debian vm, from the menu devices >>  Insert Guest Additions CD image.

Now type mount "df -h"

You should see
 "/dev/sr0 62M 62M 0 100% /media/user/VBOXADDITIONS_4.3.12_93733

Install below packages and run below commands

manny@vbox:~$ sudo apt-get install build-essential mudule-assistant

manny@vbox:~$ m-a prepare

so to to /media/user/VBOXADDITIONS_4.3.12_93733  and run

 ./VBoxLinuxAdditions.run

Note: if iso not mounted automaticall tyep "sudo mount /dev/sr0/media "

Fianlly reboot the guest.