If you just want new categories, you'll have to find the file in the presets folder called "user.mat." Duplucate that file, and rename it to whatever category and the correct extension. For example: "metals.mat" or "scifi.obp" The .mat files you should stick in the materials presets folder, the obp in the objects presets folder. Or if you've created a new one like shown above, you'll can drag it into there also. Then when you open up the presets in bryce, you will see your new category on the left side, Then hit import and import one of the oter directories. Then you can delete the ones that you don't want there.