If you're used to be a VMware user and try to switch to the Open-Source side of the Force by using VirtualBox, you may run into difficulties if you try to import an existing VDI file into VirtualBox. Actually it's quite easy, if you know how.
The main difference between VMware and VirtualBox is that VMware captures a whole virtual machine in an image, whereas VirtualBox only supports images of a hard disk. So in VirtualBox's world, you first need to create a new virtual machine, before using an existing VirtualBox image.
First copy your VDI file into VirtualBox's virtual hard disks repository. On Mac OS X it's $HOME/Library/VirtualBox/HardDisks/.
Start VirtualBox and create a new virtual machine (according to the OS you expect to live on the VirtualBox image):
When you're asked for a hard disk image, select Use existing hard disk and click on the small icon on the right:
Which will brings you to the Virtual Media Manager. Click on Add and select the VDI file from step 1.
After leaving the Virtual Media Manager, you'll be back in your virtual machine wizard. Now you can select your new VDI as existing hard disk and finalize the creation process.
Back in the main window, you're now able to start your new virtual machine:
It's quite easy, if you know how.