These will be easier to use because you will not have to create them yourselves.
We are trying to decide whether to place them in the 3D Warehouse, place them in a Component Library on your disk or both.
There is a test plant in the 3D Warehouse if you want to try it out: IRender nXt Plants

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It is 13" tall, and should have been 13' tall.
I just uploaded a new version:
http://sketchup.google.com/3dwarehouse/details?mid=277da776fdb67446...
I have tried a little bit with these trees. One thing is unclear. You can see it on my screenshot. I inserted the tree SweetGum - Liquidambar styraciflua and rendered the szene. You can see the different. Why is the component so small and the results so big (normally height) ?
Snap1Trees.jpg
Rich is getting ready to design a better interface, but for now do this:
1. Load the SketchUp Component Window
2. Enter the search term "IRender nXt Large Trees" (or Medium or Small")
3. If the collection "Large Trees" Appears, click on it to load a list of all the large trees.
4. Select and Place a tree in your model using the SketchUp component placer.
5. Render the model - the new tree should just work.