As much as i appreciate MS providing these images, they are basically worthless unless you are overclocking the crap out of your machine. Win 10 is notably slower than 8.1 on real hardware, but on a...

In VirtualBox, You just need to go to File-->Import Appliance , then select the ovf file image and click Next.
It will create everything you need from the appliance file and you can start the VM...