The Klarna Simulator is a Sandbox provided by Saferpay, which provides an extensive GUI for testing Klarna. In order to activate it, you first need to click on Configure. There you have to set a user and password, which can be anything for the simulator. However should you enter nothing, the simulator will simulate an error, as if no, or the wrong credentials have been entered. So if you want to test that case, you can do so. Then the Environment slider will simulate live and sandbox (Test) behavior, if you want, though the differences are slim. Lastly, you have to activate the countries, you want to support Klarna in. More on that later.