I've made some minor changes (see below) and it worked for me

SELECT Kurse.bezeichnung, Kurse.id, Akaadr.id;
FROM aka!akaadr INNER JOIN aka!mbship;
INNER JOIN aka!kurse ;
ON Kurse.id = Mbship.kursid ;
ON Akaadr.id = Mbship.adrid;
WHERE Akaadr.id = thisForm.pgfPageFrame.Page1.txtId.Value;
ORDER BY Kurse.bezeichnung;
INTO Cursor tmpGrid

thisForm.pgfChildGrid.Page1.grdGrid.recordsource=""
thisForm.pgfChildGrid.Page1.grdGrid.recordsource="tmpGrid"
SELECT tmpGrid
thisForm.pgfChildGrid.Page1.grdGrid.requery
thisForm.pgfChildGrid.Page1.grdGrid.refresh
thisForm.pgfChildGrid.Page1.grdGrid.setfocus


Thanks Edward Pickmann

Wolfgang Ludwig