Rewrite function from core model class

Question

I know this is a very simple question, but I couldn't get it to run, even if I read some tutorials.

I want to rewrite _saveProductCategories() from Mage_ImportExport_Model_Import_Entity_Product.

What I've done is create an empty class: app/core/local/MyNamespace/MyModule/Model/Import/Entity/Product.php with class name MyNamespace_MyModule_Model_Import_Entity_Product to extend the core model class. There I've put my implementation of the current function that I want to rewrite.

Then in the config.xml of myModule I have:

    <global>
            <models>
                <mymodule>
                    <class>MyNamespace_MyModule_Model</class>
                </mymodule>
                <importexport>
                    <rewrite>
                        <import_entity_product>MyNamespace_MyModule_Model_Import_Entity_Product</import_entity_product>
                    </rewrite>
                </importexport>
            </models>
...

But it still uses the function from the core code. What am I doing wrong?