In MacOS 2.5.2 version the following works: 1. Open Application folder (or folder where you have installed AssetForge app) 2. Right click AssetForge app and select "Show Package Contents" 3. Finder shows multiple folders, including Collections folder (in 2.5.2 MacOS build this is included) 4. Select Collections folder in Finder and add new folder (File->New Folder), name it for example "Custom" 5. Put .obj and .mtl files in the new Custom folder 6. Restart AssetForge 7. Custom folder appears at bottom of the dropdown list as a new category