From app home page

Click on “Apps”, find the “Story Mapping Boards” in the dropdown list

From project sidebar menu

Click on the “Story Mapping Board” button in the menu on the left side to open the board.

From Jira Agile board

Not only Project the app also provides story map associates with Jira Agile board. Access it by going to the board and click on “User story map” on the right corner