First of all, the meshes are in a .nif format, and the textures are .dds. You will need programs that can read these formats, and edit them seperately second, they are all compressed into .bsa archives, which you need yet another program to extract from. and finally, to actually get custom stuff into the game, you will need either the CK (not out yet) or SkyEdit. I suggest you look up the modding wiki.