Just to mention, you can also use the F6 key to bring up the material editor. Another little tip (probably mentioned somewhere in the backroom, but just in case...). If you have already created several trees of the same type, changing the material for the trunk via the material editor will affect all of them. However, if you then add another tree of the same kind, it will have the original default materials. This can be useful, but if it is not what you want, you simply need to change the material again in the editor.