Assuming you've installed CityEngine 2017.1, go to Start>Programs>Esri>CityEngine 2017.1>CityEngine 2017.1 Administrator. In the ArcGIS Administrator, click on the CityEgine folder if not already selected. For Software Product, select the option, Advanced Single Use. Then Authorized Now. Follow the authorization wizard. Provide your evaluation code when prompt to do so. This would have been sent to your email address. Once completed, you can close the ArcGIS Administrator and start CityEngine.
If the application was not installed or installed incorrectly, download the software and try the installation again.