How to open existing VMs in VirtualBox?

The previous answers appear obsolete.

As of version 4 of the Oracle VirtualBox Manager, the "Add" option is hidden in the "Machine" menu (which is an operating system menu). Just select Add, and navigate to the .vbox file you want. This works great even if the vbox file is on an external drive: if the drive is later removed, the missing boxes are simply marked as 'inaccessible'.

In the machine folder there is a *.vbox file. Drag it into the VM list in VirtualBox.

Importing existing virtual machines from another host into VirtualBox 4 and above on Windows hosts:

1. Importing the vdi virtual disk:

   1. Go to the menu: *File ⇨ Virtual media manager.

   2. Drag and drop the [vm_name].vdi into the list.

2. Adding the imported machine to the vbox manager list:

   1. Double-click on the [vm_name].vbox file in the directory of the vm, to add this machine to the vbox manager list.

   2. Double-click on the [vm_name].vbox inside the vbox manager list to start the appliance.

3. Sidenotes:

   1. If the VM comes from an older vbox release, you will have an [vm_name].xml inside a sub directory instead of a [vm_name].vbox in the same directory as [vm_name].vdi.

   2. Copy the [vm_name].xml into the same directory as the [vm_name].vdi and rename it to [vm_name].vbox.

   3. Proceed with step 1.1 or 2.1 depending on the actions you have already taken.

If you want to use an existing virtual hard disk and 'open' it into Virtual Box - you need to go to “File > Virtual Media Manager” and select ‘Add.’

Browse to your existing disk and select ok. This registers a disk with Virtual Box. Now you need to attach this disk to a new or existing VM to actually use it.