1. From Virtual Box select New
2. Select proper operating system and name your new VM. Ideally use the same name as the vdi file.
3. Select option "Use existing hard disk" and then point to the vdi file.
4. If the VM will be running a service to be accessible from outside like a web server select Bridging as the network type. You can pick any memory and CPU and so on.
When you start the new VM you might get some scary messages as the OS tries to adapt to the newer configuration (virtual hardware).
A typical problem in Ubuntu is the network will be unavailable. Just deleting the below file and restarting the VM does the trick: