Welcome to the forum! Please refer to the wiki and then go to the "Truck File Format" page and go down to props and it explains how to place/add props to a vehicle. Truck file format -
You will also need to export the mesh from blender if you are trying to use a mesh that you created. Here is the link for that: http://docs.rigsofrods.org/download/blender2ogre-0.6.0.zip
If you are trying to edit an existing mesh, here is the link to the page Editing existing meshes using Blender -
If you have anymore questions just reply back to this thread please.