Unwrap command works for mesh objects only
Sapling add-on currently create Curve-type objects which must be converted to mesh with the Alt+C shortcut before performing unwrap.
Unwrap is an operation that requires faces in order to be performed: beveled curves, text objects and other types,thought they appear as solids objects, they don't strictly "have faces". Blender makes an on-the-fly translation for us to see them, but the object really is only defined by what you can edit (curve control point, strings... but no faces!)
If confverting the curve to mesh is not suitable for your work you can consider the option to see if another kind of UV mapping can be used. See related question: Adding a texture map to a beveled curve or path