There are 2 ways to add user stories to a Sprint.
Create New User Story
On ScrumBoard Click on Button on top right corner and it will create a new user story widget like shown in example below. User can edit the Title here, select responsible and estimation value of User Story. To further edit this user story, click on ID and add further details in pop-up.
Plan Existing User Stories
On ScrumBoard take your mouse over “Add User Story” button on top right corner and it will show 2 options. Select “Existing User Story” and you will be redirected to “Planning Board” page. On this page here you can select on left panel, Product Backlog, Release or some other existing Sprint. To add user stories to the sprint, simply drag the source user stories and drop them on right panel.