If "Watapon" is added to the hand by a card effect that was activated at Chain Link 2 or higher, you can activate its effect after the chain resolves.
The effect of "Watapon" can be activated not only when the effect of "Dogu", etc. adds it to the hand, but also when it is added to the hand by drawing it with the effect of "Jar of Greed", etc.
Previously Official Rulings
These TCG rulings were issued by Upper Deck Entertainment and have since been deemed unofficial by Konami. They were previously considered official and can still be correct, as long as they follow the official gameplay rules, or unless Konami has issued a ruling that says otherwise. References: