URDF and URI's

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Hello all, I am having some trouble loading a model supplied by a colleague in rviz. For my own interests, I wish to be able to load a urdf which is not part of a ros package. Thus, everywhere where it was written:

 <mesh filename="package://derp/meshes/zmp.stl" />

I replaced with:

 <mesh filename="file://home/constantin/workspace/OpenRAVE/data/cRoNA/meshes/zmp.stl" />

and then afterwards

 <mesh filename="file:///home/constantin/workspace/OpenRAVE/data/cRoNA/meshes/zmp.stl" />

Well both of those didn't work, I still got the following errors:

[ERROR] [1323810997.737214696]: Error retrieving file [/home/constantin/workspace/OpenRAVE/data/cRoNA/meshes/zmp.stl]: <url> malformed
[ERROR] [1323810997.737386409]: Could not load model 
     '/home/constantin/workspace/OpenRAVE/data/cRoNA/meshes/zmp.stl' for link 'zmp': OGRE 
     EXCEPTION(6:FileNotFoundException): Cannot locate resource  
     /home/constantin/workspace/OpenRAVE/data/cRoNA/meshes/zmp.stl in resource group 
     Autodetect or any other group. in ResourceGroupManager::openResource at /tmp/buildd/ros-
     electric-visualization-common-1.6.2/debian/ros-electric-visualization-
     common/opt/ros/electric/stacks/visualization_common/ogre/build/ogre_src_v1-7-
     3/OgreMain/src/OgreResourceGroupManager.cpp (line 753)

Could anyone give me some tips on what I am doing wrong? A link to expanded documentation would be great as well, the Resource Retriever page is quite empty. Thank you so much!