The Simulator is the tool that allows you to test dynamic creatives and the paths created with the Journey Designer and the Storyboard in a simple and effective way.
Below a guide below that explains how to open it and how it works.
Once you created all your stories and applied a rules engine at the campaign level, you can preview how the stories flow.
...
2) All stories need to be published. For each story click on edit and under "Workflow" select "Published" instead of "Draft"
The Simulator is the tool that allows you to test dynamic creatives and the paths created with the Journey Designer and the Storyboard in a simple and effective way.
Click Once you've opened the preview, click on the symbol 
at the right top to open the dialog window window and simulate the change of context via the dropdown list.
Press 
to confirm the change of context and simulate another impression.