This sample project demonstrates how to edit report in the designer. First of all, create new StiReport():

private void ButtonNew_Click(object sender, RoutedEventArgs e)
{
	StiDesignerControl1.Report = new StiReport();
}

...

Then, use StiDesignerControll.Report to attach report to the designer:

...

private void ButtonEdit_Click(object sender, RoutedEventArgs e)
{            
	var report = new StiReport();
	report.Load(@"Reports\SimpleList.mrt");
	StiDesignerControl1.Report = report;
}

Also, you can use DesignV2WithWpf() to edit the report:

...

private void ButtonV2_Click(object sender, RoutedEventArgs e)
{
	var report = new StiReport();
	report.Load(@"Reports\SimpleList.mrt");
	report.DesignV2WithWpf();
}

...

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