Though the problem itself was trivial (forgot the path) I leave question and answer in case someone else needs the same functionality.
let path = NSBundle.mainBundle().resourcePath!.stringByDeletingLastPathComponent.stringByDeletingLastPathComponent
let task = NSTask()
task.launchPath = "/usr/bin/open"
task.arguments = [path]
task.launch()
exit(0)
Edit (daily Swift syntax change for Sw3; works also for Sw4):
let url = URL(fileURLWithPath: Bundle.main.resourcePath!)
let path = url.deletingLastPathComponent().deletingLastPathComponent().absoluteString
let task = Process()
task.launchPath = "/usr/bin/open"
task.arguments = [path]
task.launch()
exit(0)
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