Hi Alex Durrant,
I am afraid that we cannot determine where the problem is with your current setup without receiving any additional information.
We have prepared a sample application which demonstrates data insertion. Simply select a parent row and press the button.
Hope it helps.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items