The BR3 extension is for Bryce 3D files (what gets saved when you hit CTL S) BR4 is the same but for Bryce 4 files. .mtl files are material files and must be imported through the presets library. .obp are Bryce Object Preset files and must be imported through the preset library as well (click on the triangle to the right of the word Create) .bsk files are Bryce sky files and must be imported through the Sky library presets. Hope this helps. Sarah