More info you can find at wiki
- Clone this project
 - Import project structure from Gradle
 - Update remote repositories if need 
Settings | Build, Execution, Deployment | Build tools | Maven | Repositories
select bad repository and click onUpdate - Run "stepik-union" run configuration
 
- 
Just install Stepik Union plugin from plugin repository.
Default settings | Plugins > Browse repositories... - 
Find Stepik Union plugin and press Install.
 - 
To support Python code step in plugin, install also Python plugin from repository.
 
And then restart IDEA.
- Set up Python SDK, if you install Python plugin.
 - Open 
Default settings | Other Settings | Stepik, authorise in this window. PressOk - Create new project with "Stepik Union" module type.
 
To send your code to the platform use actions in the "Step Description" tool window.