Power on, Press Cmd+R to enter recovery mode.
Connect to WiFi
Apple should then download enough software to
install an operating system.
If you attempt to install but do not see a
disk to install the operating system onto, then the following is possibly the
problem.
The problem is that you have a virtual disk
but still don't have any partition on it (disk in terms of Windows filesystem).
To fix that do the following:
1. Go to menu Install
OS X Mountain Lion -> Quit Install OS X.
2. Select Disk Utility
3. Select your disk on
the left side. If you can't find the disk, click on the button near the top
left, then select the "Show all disks" option.
4. Select erase tab on
the right side
5. Format: Mac OS
Extended (Journaled)
6. Name: Give it a
name
7. Click erase. This
will create a partition on your disk drive
8. Quit Disk Utility
and continue with installation as you did.