Toad Data Modeler doesn't display marker Not Null marker when using IDEF1X notation.
This is not a defect, it was designed that way, however a new enhancement request has been created to implement Not Null marker into IDEF1X notation.
This enhancement request will be considered in future version of Toad Data Modeler.
In the meantime, IE Notation shows a workaround for this situation, Not Null marker can be seen in the image below:
