EditorUtility.OpenFolderPanel  

static function OpenFolderPanel (title : String, folder : String, defaultName : String) : String

Description

Displays the "open folder" dialog and returns the selected path name.

See Also: OpenFilePanel, SaveFolderPanel functions.

Open Folder Panel.

// Opens a directory dialog for copying all the png textures
// in the directory to where the current open scene resides

import System.IO;

@MenuItem("Examples/Load Textures to directory")
static function Apply () {

    var path = EditorUtility.OpenFolderPanel("Load png Textures of Directory", "", "");
    var files : String[] = Directory.GetFiles(path);
    for(var file : String in files)
        if(file.EndsWith(".png"))
            File.Copy(file, EditorApplication.currentScene);
}