The Audiomatic Rack Extension is included free in Reason version 7 and later. When buying and registering Reason 7 or later, a license for Audiomatic is automatically issued to your user account. To download Audiomatic, login in to Your Account, and click the Sync All button.
Note: In Reason 9 the Audiomatic Rack Extension is embedded in the Reason 9 installation file, and doesn’t require a separate download. See here for more information.