Windows Server 2012 Adding .NET Framework 3.5 Feature, gives error message "The source files could not be found". Error

If you have Windows Server 2012 iso file or DVD, you can install .net framework 3.5 on Windows Server 2012 easly.
1. You have to insert Windows Server 2012 DVD or map iso to D drive.
2. Open Add Roles and Features Wizard
3. Select .NET Framework 3.5 Features and then click next,
4. Select Specify an alternate source path link in the Confirm installation selections screen.
5. Path: D:\Sources\SxS and then click OK.
6. Finally click Install button.

Ref: http://social.technet.microsoft.com/Forums/en-US/winserver8setup/thread/5a786052-38a1-46f9-add2-e6487bc6a8aa/