Go to your VirtualBox.app and Right Click and choose:
Show Package Contentsand the package should display
Navigate to the MacOS folder & Drag the VBoxManage command to your Terminal.app window
drag the dmg file we created with super duper tigerxxx.dmgto the Terminal.app window
then type in the name of the VDI image you wish to create:
(add the .vdi)
The VDI image will be created in your HOME folder, move it to where you feel would be good...
HOME/Library/VirtualBox/VDI might be a good place