The itemis Model Viewer trial version comes with an evaluation license that is valid for 30 days. If you want to continue working with itemis Model Viewer beyond that period of time you have to acquire a license.
The license manager documentation describes how you can install a license file or configure a license server for floating licenses. To access that documentation, select Help → Help Contents from itemis Model Viewer’s main menu, then open the YAKINDU License Management documentation.