The TGO format objects that Greenworks allowed us to publish (in the free plants bundle) should load up just fine by default, no tweaking necessary. As long as you don't change any of the settings in them then they should render right off the bat. You may find they're small compared to your scene, but they're probably at the right scale, it's just that the scene is much bigger than it seems. You can either scale the objects up or move your camera in closer to see them.
As far as .mtl files, this has almost nothing to do with TG2 since you can't create nor manipulate them in TG. You will only need to do anything with the .mtl file if you're working with an OBJ format object that has not already been made available in TGO format. This would be virtually any object that is not from the Greenworks bundle we provided and is in OBJ format.
There are separate programs that deal with manipulating .mtl files, or you can load them in a text editor. TG2 simply reads the information that's there and assigns textures based on it, but you can't manipulate the .mtl or texture placement from within TG2 at this time. The .mtl will be automatically loaded if the .obj and .mtl are named and referenced properly, which any decent 3D program should do automatically on export.
The next update will also include some changes that make it easier to work with manually assigning textures to objects without properly setup .mtl files.
The grass object comes with a Default Shader already attached. To adjust color simply change the Color in the Default Shader. You can also use a Default Shader to provide basic color to an imported object in the same way, connecting the Default Shader to the Shader input inside the Object node (right-click->Internal network). A Power Fractal with different "high" and "low" colors could provide more variety and would be connected in the same way.
- Oshyan