Open Whisper Systems has finally released Signal 2.0 which now also includes TextSecure functionality. Blog post: https://whispersystems.org/blog/the-new-signal/ FAQ: https://github.com/WhisperSystems/Signal-iOS/wiki/FAQ
If you're in the main Conversations window, click the + icon and select a contact to send them messages. If you mean SMS with text messages, iOS doesn't give 3rd party apps access, it's data messages only.