Click the PSCAD tab in the ribbon control bar, hover the mouse pointer over Open, and select Import Project.
The Open File dialog window will appear with v4.1 or v4.2 default file types (*.psc, *.psl). Navigate to the desired project and select it so that its name appears in the File name field. Click the Open button to import the project. The project should appear in the workspace window, indicating that the project was indeed imported.
Importing a project, as opposed to loading one, is the act of both loading and converting an older style project (i.e. *.psc or *.psl) into the new XML-based file format (*.pscx/*.pslx). For more information on the import process and some possible pitfalls see Converting PSCAD v4.1 and v4.2 Projects to X4 in the chapter entitled Migrating from Older Versions.
Projects may also be imported via drag and drop. See for Loading Projects and/or Workspaces more details.