When the RadRichTextBox property IsReadOnly is set to 'True', the only way to add comments to your document is through the InsertComment() method from your application's code-behind. The mentioned method receives Comment objects.
Of course to be able to see these comments, the property ShowComments of the RadRichTextBox control must be set to 'True' also.
Hope this helps.
All the best,
the Telerik team