NSOpenPanel *panel = [NSOpenPanel openPanel];
[panel setCanChooseFiles:NO];
[panel setCanChooseDirectories:YES];
[panel setAllowsMultipleSelection:YES]; // yes if more than one dir is allowed

NSInteger clicked = [panel runModal];

if (clicked == NSFileHandlingPanelOKButton) {
    for (NSURL *url in [panel URLs]) {
        // do something with the url here.
    }
}