I had this problem, even though i had created a virtual drive.
I realised that, you then have to go into Utilities->Disk Utility. Your 3 or 6GB virtual disk should be visible here. You then need to select it and create a partition on it, then choose to format it as MAC OS Extended (journalled). Takes about 10 mins on a 6GB virtual disk.
Once you've done this, an available hard disk will magically appear to on the installer!!