Sometimes it can take a while for the watch face to appear (up to an hour or 2) in the Android Wear app after you have purchased from Google Play. Not sure whats causing it but it’s not related to the watch face in itself but rather the installation process.
So if something go wrong in installation its usually recommend to re-install it using those steps:
1. Disconnect devices (watch and phone)
2. Uninstall the watch face
3. Restart watch and connect device again
4. Then finally re-install the watch face
That should start it right away, and that tip works for pretty much all apps Android Wear related.