What you have to do is use the OpenBRF editor - the old BRFEdit wont work with warband. There is a link to download it around here somewhere.
* Start OpenBrf.
* Click on "import" at the top.
* Click on "Anything from a brf" at the bottom of the pop-down list.
* Select the brf from the Mount&Blade mod you want. That should import all of the items in that brf to OpenBRF.
* Save as your new brf.
* You will, of course, have to do the same thing for your materials and textures - for warband I put them all in one brf for convenience.
* The rest of the python stuff is pretty much the same as M&B.
Hope this helps! This problem had me stuck on Warband modding in the beginning as well!