In the project navigation bar, select Code. It takes you to an outline of your project structure.
Drill down to see files in a directory, and select a file to see its code.
If your project is too large for easy exploration via drilling, the search feature on this page will help. While the global search in the main menu returns results from throughout the SonarQube Server instance, the localized search on the code page is restricted to files and directories in the current project.