Where/How to generate the validation key and decryption key for SSRS View State Validation According to this Technet article, I need to :

Generate a validation key and decryption key by using the autogenerate functionality provided by the .NET Framework.

The step does not explain how to generate the validation key and decryption key.
You can generate the machine key through IIS Manager.

Use Server Manager -> Roles -> Web Server (IIS) -> IIS Manager. Click on the Web Server in the IIS Manager, then in the right pane, double click Machine Key in the ASP.NET section.