Thanks for contacting with iToo Software and hope you enjoy our plug-in.
Actually, 2D billboards are not supported by Corona Renderer.
As for proper use of 2D billboards the Forest Material should be used, but in case of Corona Renderer this is not supported, there is no material library for these. There are only few features, that are not full supported on this render engine (please feel free to check the Feature Compatibility table in our on-line reference documentation
Anyway, if you decide to give it a try in Corona, please do the following:
Set ForestPack Display->Render to mesh:
Create a Plane object in Front viewport- the size as desired tree set pivot to its bottom.
Copy Your current unsupported material to slate editor.
Apply Standard material to Your newly created Plane object (Not Forest Material, but Standard.)
Then go to forest Pack and pick Your created plane as custom object.
Hope that helps and in case of any further question or doubt please don't hesitate to contact us. Have a great day.