Hello Urs Graf,
Thank you for contacting Telerik Support.
Running Manual tests through the Test tab via "Start manual test" button will not populate the Results tab unless you select Reset Results on the 4th step - it will reset all current test results and give you the option to save the results file. If you save the file in the Project\Results folder and after that go to the Results tab and click Reload button, you should be able to see the results in the Test Results Calendar.
Another approach is to include one or more Manual tests in a Test List, and then execute the list.
Once the Calendar view on the Results tab is populated with result rectangles, you can double click on one of them and this "drills down" into the result to load it. See this short video for more information.
I hope this helps.