VMDK/VMX are VMWare file formats but you can use it with VirtualBox:
- Create a new Virtual Machine and when asks for a hard disk choose "Use an existing hard disk"
- Click on the "button with folder and green arrow image on the combo box right" which opens Virtual Media Manager, it looks like this (you can open it directly pressing CTRL+D on main window or in File > Virtual Media Manager menu)...
- Then you can add the VMDK/VMX hard disk image and setup it for your virtual machine :)