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