Tools/OgreXml/install Ogre Blender xml in the sdk asks the user to point to the blender scripts directory.
Blender wasn’t proposing mesh.xml export in the addons of blender after that.
So, I moved it to the blender script/addons directory, and then, I could select the addon in Blender. And then I could export to mesh.xml.
So I guess the install Ogre Blender xml window should ask to be pointed towards the Blender scripts/addons directory instead.