Hi Atanas,
thank you for your quick help. I could only make it work partly 
a) I was able to add the checkbox column and in the running app check/uncheck the rows
b) But I was not able to utilize the grids property "value" as in my version this seems not to be there (see screenshot). Also I guess I have to add and remove items from "selectedItems" when the checkbox of the rows is changed or databind it. Is it done in the eventhandler of the checkbox or is that somehow automatic?
Best regards,
Frank