I've tried TDM3 on XP 64bit only to discover that it wasn't compatible. I have since upgraded to Vista and used TDM to export a fairly large model to a DMX file and tried importing it. It looked like it was importing all the enties without a problem, but it fell over when it got to the relationships saying "XML cannot be read from file".
In this instance, the version the user was running on does not support XP 64 bits nor Windows Vista. However, TDM v3.1.5 which can be download at https://support.quest.com/download-product-select and does support Windows Vista. In fact, TDM 3 allows you to open DM2 files and DMX files of Toad Data Modeler 2. (DMX file is a DM2 file in XML format.) This applies only to models of database systems that are supported by both applications - Toad Data Modeler 2 and Toad Data Modeler 3.
It is not possible to open models created in Toad Data Modeler 3 in Toad Data Modeler 2.