To activate the RadBusyIndicator control you have to set its IsBusy boolean property to True.
Setting the IsBusy property
RadBusyIndicator also gives you the IsBusyIndicationVisible property. It is a read only boolean property which value will be equal to True whenever the RadBusyIndicator is enabled and the busy content is still visualized.
To read more about the possible scenarios where you can use the IsBusyIndicationVisible property, look here.