This example shows how to load a report template and edit it in the designer. You need to create the report object of the StiReport type, then load the report template file by calling the loadFile() method. To edit report in the designer, you can simply assign a report object to the report property of the designer:
// Set the full screen mode for the designer
var options = new Stimulsoft.Designer.StiDesignerOptions();
options.appearance.fullScreenMode = true;

// Create the report designer with specified options
var designer = new Stimulsoft.Designer.StiDesigner(options, "StiDesigner", false);
// Create a new report instance
var report = new Stimulsoft.Report.StiReport();
// Load report from url
// Edit report template in the designer = report;

In the screenshot below you can see the result of the sample code:

Edit Report Template in the Designer