I am using 32 bit Kali 1.0.5, but I think this should be the same for the 64-bit version. Here are the steps I use to install the virtualbox guest addons on a fresh install of Kali (if you have made modifications it might not work):
1. Download the linux headers to compile the virtualbox modules with this command:
apt-get update ; apt-get install linux-headers-$(uname -r)
2. In virtualbox click on the devices menu and click "Install Guest Additions" at the bottom. You have to install this way and not using apt-get because the package in the repositories never works right.
3. In Kali's places menu you should see the virtual CD. Open it to be sure it is mounted.
4. Open a terminal and run the following (as root):
cd /media/cdrom
bash VBoxLinuxAdditions.run
5. Reboot. When Kali restarts your guest additions should be installed (you can tell it works if you can resize the window and the resolution of the VM changes to match the window).
“Nothing in this world can take the place of persistence. Talent will not; nothing is more common than unsuccessful people with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. The slogan 'press on' has solved and always will solve the problems of the human race.”