This page covers the basics of using models in Unity. In general, you can do anything with them that you can with any static mesh.
To add models one at a time, simply drag the SpeedTree object from the “Asset” area in Unity and drop it in the scene. The model can then be transformed like any other object in Unity.
It is important to note that rotating an instance of the asset is fine. The billboard shader will take the rotation into account when selecting the appropriate angle from the 360-degree billboard atlas.
SpeedTrees can be painted on terrains similar to other mesh objects. Go to the tree tab on the terrain and click “Edit Trees”. Add the tree asset you wish to paint in the resulting add tree dialog, and it will then show up in the “Trees” section.
After selecting the tree you wish to paint, you can adjust the painting tools with the parameters below it. “Tree height” lets you control the scaling of the tree when painted. With SpeedTrees, you most likely want to enable “Lock width to height” and “random tree rotation” for the best results (these are enabled by default).
You can adjust the switch from 3D tree to billboard with the “billboard start” and “fade length” parameters on the settings tab of the terrain.
Wind behavior is tuned inside of the SpeedTree Modeler. To see the wind animation in Unity, you must add a 'Wind Zone' object to the scene. Use this object to set the strength and direction of the wind.