http://www.apple.com/support/appleid/
https://discussions.apple.com/community/notebooks/macbook_pro
To install OS X Lion or Mountain Lion on another Mac, open the Mac App Store, sign in to your account, and re-download the OS by selecting it in the Purchases list. Follow these steps:
- Download Lion or Mountain Lion from the Mac App Store on one of your personal authorized Macs. The download is approximately 4 GB in size.
- Install the OS.
- Log in to another computer that you wish to install it on.
- Open the Mac App Store. Enter your Apple ID and password that was used to download either Lion or Mountain Lion on the first computer.
- Click the Purchased icon, then click the download link for the appropriate OS.
- Run the Installer once it has finished downloading to the new Mac.
https://discussions.apple.com/community/mac_os/os_x_mountain_lion?view=discussio ns
I suggest you download it once, save the installer ESD package first before installing, and keep that to use again. You can copy it to flash drive and make an OS X installer too.
- Create a bootable install drive for OS X Mountain Lion, make one manually with a USB drive or use the LionDiskMaker tool to automate the process with a USB or DVD
- With the boot installer drive connected to the Mac, reboot and hold down the Option key
- Choose the “Mac OS X Installer” startup volume from the boot menu
- Select “Disk Utility” and choose the hard drive you wish to format, click the “Erase” tab, and then pull down the “Format” menu and select “Mac OS Extended (Journaled)” as the type, name the drive if you wish
- Click the “Erase” button and let the drive format – this is the point of no return
- When finished, quit out of Disk Utility and now select the “Install Mac OS X” option from the menu
- Choose your freshly formatted hard drive and install Mountain Lion
When the Mac reboots you will have a clean installation of Mac OS X 10.8 to work with.
Another reliable method:
After you’ve downloaded the OS X Mountain Lion installer from the Mac App Store, launch LionDiskMaker and it will locate the installer app, extract the disk image, and then make the boot disk. It’s about as simple as it gets.
For USB install drives or SD cards the drive needs at least 8GB of space available. LionDiskMaker formats the drive you point it at, remember that when you are picking a disk to make bootable for the installer.
Get the newest version of LinoDiskMaker free from the developer