First, Unity compiles Plugins folder.
So, Library should be there.
http://docs.unity3d.com/Documentation/ScriptReference/index.Script_compilation_28Advanced29.html
To find UISprite class declaration,
I put NGUI folder under Plugin.
It worked!
...
edit:
This settings cause unity-editor error like this.
http://forum.unity3d.com/threads/159001-I-put-NGUI-folder-to-Plugins-editor-error-happens.-(NGUITransformInspector-couldn-t)
So, I changed directories like this.
http://d.hatena.ne.jp/shinriyo/20120606/p2
(Japanese)
