They've made it even simpler.
Just make sure you've mount the Guest Additions via Devices. Then you'll see a "Guest Additions" CD icon on your desktop.
Just right-click, and select the run with Autorun Prompt. And a new window will pops up, and installing the necessary files.
You'll have to restart your virtual computer to take effect.
or, via terminal way
sudo sh [filename].run
or was it
restart ubuntu by
*Note* Some functions might now available be available in Ubuntu VB
-mode, compared to actual working/independent machine.
Also, remember to blacklist this entry, to fix the slow/flickering screen refreshes.
Open up your terminal
sudo gedit /etc/modprobe.d/blacklist-framebuffer.conf
Enter the following line to the end of the file
Save, and restart Ubuntu.