I am running Ubuntu Server in a Windows 8 host. When I click the "Devices>>Insert Guest Additions CD image", I am greeted with a prompt that says: "Unable to insert the virtual optical disk : Would you like to try to force insertion of the disk?" What should I do with this? Should I "Force Mount" or "Cancel"?
You don't insert Guest Additions CD image you don't need to at all.
open up a terminal and run this command to open up additional drivers.
sudo /usr/bin/software-properties-gtk --open-tab=4
Using x86 visualization solution
Then click on apply Changes
If you wanted to Install this in terminal you would need to run this command.
sudo apt-get install virtualbox-guest-dkms