If you already have the Guest Addition ISO disregard the next bit.
One of a few ways is to download Guest Additions from the link below and download VBoxGuestAdditions_4.1.20.iso
(if you are not using VBox version 4.1.20 go to appropriate folder and download it from there) http://download.virtualbox.org/virtualbox/4.1.20/
After you've downloaded it, load VBox and go to Settings
for your MiniPCLOS KDE 4.83 VM and choose Storage
- then add the Guest Additions ISO as you would any other)
Start your MiniPCLOS KDE 4.83 VM and when the desktop has loaded go to Devices > Install Guest Additions
If you don't get any prompts, open Dolphin and in the Places panel you should see an entry for the Guest Additions ISO
Select the ISO and a list of files will load - please click on the one named autorun.sh
A Konsole window will appear and a second later another window will open where you'll be prompted to enter the root password - do so and then let it run it's course. At the end it should say something likePress Return to close this window...
- do so, and the Konsole window will close.
Reboot and you should be good to go