[ubuntu] Virtualbox - virtual harddrives - not available

Hi guys, I'm completely new to Linux, I need to use it now to use OpenFOAM, so I'm having a go at installing Ubuntu onto a vm using VirtualBox. Basically I've followed the setup to create a VM, and then installed using ubuntu-12.10-desktop-amd64.iso. Now my problem is that the virtual hard disk I allocated to the vm is 25gb, but when I log in, and go to Computer, I can't access the hard disk. It gives me "unable to mount location, can't mount file". Is there something else I'm supposed to do, because I've searched everywhere and people only talk about external hard drives...

The virtual drive is the default format .vdi, I'm quite certain I'm missing something here but just don't know what. Sorry for being a complete noob but I really have no idea what's going on...