Encore is no longer in active development. Encore CS6 was the last version released.
The CS6 applications are still available are part of a Creative Cloud membership. Encore is installed by first installing Premiere Pro CS6, which includes Encore CS6.
This document describes the process of installing Encore CS6.
Note: The ability to install CS6 applications is only available to paid Creative Cloud subscribers and not available with trial memberships. It is not possible to install Encore CS6 with a trial membership to Creative Cloud.