Re-install Mac OSX Lion on my Macbook Pro 2011

Erase and Install Snow Leopard

1. Boot from your Snow Leopard Installer Disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu.

2. After DU loads select your hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Note the SMART status of the drive in DU's status area. If it does not say "Verified" then the drive is failing or has failed and will need replacing. SMART info will not be reported on external drives. Otherwise, click on the Partition tab in the DU main window.

3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the format type to Mac OS Extended (Journaled.) Click on the Partition button and wait until the process has completed.

4. When process is completed quit DU then complete the Snow Leopard installation.

Unfortunately, this is the only option you have unless your computer came with a Lion USB installer flash drive, you can restart to the Recovery HD, or saved a downloaded Lion installer.

If you can boot to the Recovery HD then:

Install or Reinstall Lion from Scratch

If possible backup your files to an external drive or second internal drive.

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

Erase the hard drive:

1. Select Disk Utility from the main menu and click on the Continue button.
2. After DU loads select your hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Note the SMART status of the drive in DU's status area. If it does not say "Verified" then the drive is failing or has failed and will need replacing. SMART info will not be reported on external drives. Otherwise, click on the Erase tab in the DU main window.
3. Set the format type to Mac OS Extended (Journaled.) Click on the Erase button and wait until the process has completed.
4. Quit DU and return to the main menu.

Reinstall Lion: Select Reinstall Lion and click on the Install button.

Note: You can also re-download the Lion installer by opening the App Store application. Hold down the OPTION key and click on the Purchases icon in the toolbar. You should now see an active Install button to the right of your Lion purchase entry. There are situations in which this will not work. For example, if you are already booted into the Lion you originally purchased with your Apple ID or if an instance of the Lion installer is located anywhere on your computer.