The "On-site Messaging" tab of the module contains configuration options and settings for Klarna On-site-Messaging.
The Client Identifier is found in the Klarna Merchant Portal, on the Installation page of the On-site Messaging app, under "Conversion boosters" in the top menu. Ensure you copy it without quotes or whitespace. Then paste it in the Data Client ID field in the PrestaShop Klarna module.
Next determine which placement types are desired (from the Placements page) and add those placement keys in the shop's placement table; product and cart page placements are recommended.
Data from the customer's browser for the shop (e.g. language and currency switcher) and shop data are used to determine the appropriate data-locale for the On-site messaging placement; more details are covered the User Guide.