I created an extension with a domain model Message. This model has a relation m:n with the TYPO3 pages (the one which has the details of the pages,
I think you have to write the mapping with camel case letters (the class name). Although this post is in German, I think the code might help you. The author added some fields he is going to use to the class and also added a mapping in the typoscript of the extension (see the example code there). The most important part of the German text is that this example there was designed only to read from the db. If you want to create new pages using the model, you have (at least) to add the TCA and setters in the model class to make it work.