If you find difficulties installing guest additions via the VM "Devices" menu, you can easily follow this tutorial which will help you use the command line under Ubuntu/Linux Mint running as guest to install VirtualBox "Guest Additions", which is a set of tools and system applications that allow the user to copy/paste between host and guest, enable full screen resolution, and it also includes device drivers (graphics card, keyboard, wireless cards, etc.) to ensure better usability and performance for the guest OS.
The tutorial is workable for Ubuntu 12.10/12.04/11.10 or older (applicable also for Ubuntu derivatives such as Kubuntu, Xubuntu, etc.), and any Ubunu-based system like Linux Mint 13/12 or older. Open the terminal and install VirtualBox guest additions with these commands:
sudo apt-get install virtualbox-guest-additions
sudo apt-get install virtualbox-guest-dkms virtualbox-guest-x11 virtualbox-guest-utils
After installing all packages mentioned above, restart the virtual machine to enable guest additions.