Don't modify .vbox or others files !
Follow these steps :

Just tried with version : 4.2.18

Right-click on VM and select Forget saved state (Ctrl+J) if the VM is saved
Go to the Configuration (Ctrl+S) of virtual machine
Select Storage
Click on icon Add Hard Disk (IDE or SATA)
Select Choose an existing disk
Select the file *.vdi created for your virtual machine, in the right folder
Valid with OK
Start your virtual machine