If you want to add an OBJ file in Unity for 3D modeling or game development, here's a quick guide to help you out. First, make sure you have the OBJ file ready that you want to import. In Unity, go to the Assets folder in the Project window. Right-click on the empty space and select 'Import New Asset'. Find and select the OBJ file from your computer, and Unity will automatically import it into your project. Once imported, you can drag and drop the OBJ file into the Scene view to add it to your scene. You can also view and configure the OBJ file's properties in the Inspector window. With these simple steps, you can easily add an OBJ file in Unity and start using it for your 3D modeling and game development projects.