Hello Becky,
we're sorry you're having trouble with TDM but I'm not able to reproduce this error. Could you send us a whole error report? If the error occurs again, please click on the link in the error message to see a whole call stack, check the option "Copy to clipboard" and paste it to a new text file. Or send us your TDM.el file (Eureka Log generated file with default location on c:\Users<USERNAME>\AppData\Roaming\Quest Software\Toad Data Modeler\TDM.el), please.
You can also perform a model test (go to the Setting | Option | General and enable Expert mode), right-click on a model name in Physical Model Explorer or Application view , select "Test Model" and see the results in Message Explorer.
Regards,
David