To add books to your Schooltas account you need to add a bundle/license code. Ask your vendor how you can get your personal bundle code.
To add the bundle code, open the settings menu in your dashboard and select the menu 'my bundles'.
In the next window, you can add the bundle code you have been given by your publisher or vendor.
If you want to try out Schooltas you can also add a demo bundle. To do so, tap the button "add a bundle code" in the "my bundles" window.