Thanks for the message. The offer appears randomly in the app, but if you're sure its not appearing, maybe you're already on the pro version. You can check and confirm this by using pro features like subtitles download, chromecast, airplay, passcode etc. If you're able to use them you're on the pro version.
Pleased to hear that you considered this app. At the moment the repeat feature isn't available with chromecast as is the subtitles feature. The app is being improved constantly and you will see more features added in the coming updates.