As Koush has noted on his Google+ account, the app has been submitted to Amazon for App Store approval but if you're not wanting to wait for that and don't mind handling a bit of sideloading, you can download the beta .apk file for it right now.
The instructions are fairly simple. Sideload the AllCast receiver APK on your Fire TV, then start up AllCast or Mirror on your Android. Your Fire TV should show up and be a cast/mirror target. You can download the app via Koush's Google+ page.
Source: Koushik Dutta