Installing Mac OS X Mountain Lion (10.8) on VirtualBox

Please follow the following steps:

1. Purchase Mac OS X 10.8 Mountain Lion from Apple's Web Store. 

The downloaded installer should be a DMG file and when you mount it, you can see its content as in the following screenshot:

Some installer DMG file can't be mounted from Virtualbox because it contains only one file, an APP file. The APP file is used to reinstall (or do clean install) of existing Mac OS X you're currently on. You cannot use it as a bootable DVD. Hence, make sure you get the right one and I'm not sure why there are two different kinds of this file.

2. Create a new virtual machine the following settings:

- Set video memory to 60 MB
- Set RAM to more than 2048 MB, for example 2653 MB
- Create a new optical disk (CD/DVD ROM) and mount it to the installer (DMG) file. Don't forget to check Live CD/DVD checkbox
- Make the CD/DVD ROM drive is the first in boot order

3. Start the virtual machine and follow the instructions on the screen. 

There will be a screen asked you to select a disk to install the OS on but there won't be any disk to select. You need to format the virtual hard drive:
- Select Utilities then Disk Utilities... 
- Select the disk in the sidebar on the left and then click on Erase tab in the sidebar on the right
- Choose the file format as Mac OS Extended (Journaled)
- Click Erase button
And you'll now see the disk in the "select disk" screen and then follow the instruction on the screen.

4. It will ask you to restart after the installation's finished. 

You might get an error when it starts again. You need to turn off the virtual machine and make it boot from the virtual hard disk instead of the virtual CD/DVD ROM drive.

When you boot from the virtual hard disk, you might get stuck as in the following screenshot. If it takes you too long to wait, you might need to shutdown the machine and start it again and then it should work this time.