Make sure your iPhone is connected to your Apple Watch and it is updated to iOS 10 and watchOS 3.
Open the Apple Watch app on your iPhone, scroll down and tap on the Runtastic app you would like to install (Runtastic, Six Pack, Butt Trainer or Leg Trainer).
Make sure to use the latest Runtastic version (7.0 or higher).
Turn on “Show App on Apple Watch”. After a short installation process, you'll find the app on your Apple Watch.