Toad Data Modeler version 3.3 will allow users to generate XSD files for their models, however, those XSD files just describe structure of *.TXP files. Try to save a physical model -> it will be saved in TXP format. Rename the TXP file to XML and open it in XML editor. You will see XML file structure. The generated XSD describes structure of this XML format.
Also, it will be is possible to generate simplified XML file from Toad Data Modeler version 3.3 (via Model | Reports / XSL Transformation…). We will offer users simplified XML files and also XSD that will describe the structure of simplified XML files.
Both features, both types of XSD files will just describe the structure of XML files. You will not be able to generate XSD for your entities, attributes and so on.
But… Few weeks ago I created an XSLT file that allows one of our customers to generate XSD for entities, attributes, domains… This template was based on his requirements and I am not sure if the same template will be useful for you. If you can, please send me more information about what you want to model in TDM and what you want to generate. What would be perfect is a sample model and a file with output you wish to generate. Please send us both files to email@example.com. Thanks!