Here's how you can install Linux Mint 12 in VirtualBox.
1. Download and install VirtualBox on your system.
2. Download the Linux Mint 12 ISO file.
3. Open VirtualBox.
4. Click the New button and follow the steps in the Virtual Wizard to set up your virtual machine.
5. Before starting your new Linux Mint 12 virtual machine, click the Settings button.
6. Go to Display and set it to use 128 MB of video memory, then click the "Enable 3D" check box.
7. You can now start your Linux Mint 12 Virtual Machine.
8. After Linux Mint boots into its live desktop, begin the Linux Mint install routine by clicking on the Install icon on the desktop.
9. Finish the install and then restart your virtual machine. Make sure the ISO file is unmounted so Linux Mint 12 will boot into your installed desktop and not the live desktop.