How can I import my old OS on VirtualBox?

A .vdi (virtual disk) file is a disk image used by VirtualBox. You can think of it as representing one piece of your virtual hardware--the hard disk.

If you only have one piece of the machine, that's the important one--it's a virtual machine, after all, and new virtual parts don't cost anything.

Open Virtual Box. Create a new virtual machine. When you get to the point where are asked about its hard disk, specify that .vdi file as the disk. Or make a virtual machine and, later, add it as a disk and remove whatever unused virtual disk was originally created for it. (If you do it that way and you want to delete the new blank disk, make sure you don't delete the .vdi file that has your data on it by accident!)

By the way, if you can manage to recover the whole virtual machine (i.e., the other files from it as well), then you can import it into VirtualBox by opening the virtual machine file (ending in .xml).

_{Your question may be considred off-topic, as you're asking how to reconstruct a VirtualBox VM in OS X. Which operating system the VM is running (in this case Ubuntu) is irrelevant, so this isn't really about Ubuntu. On the other hand, VirtualBox is also Ubuntu software--for Ubuntu users who want to have virtual machines. Answers to your questions won't really be host system specific either, so I suppose this might be considered on-topic, which is why I've posted this answer. Better sites for it may be Ask Different and Super User, but please don't have the same question open on more than one SE site at a time.}