How do I enable those colourful symbols I see other people using? (Emoji)

Those colourful symbols are known as Emoji. If you are running iOS 5, enabling the Emoji keyboard is easy.

Here's an article on CNET describing how to enable Emoji for devices running iOS 5.

Note that enabling the Emoji keyboard will enable it for all apps, not just Feathers. Also note that only people on iPhone, iPad, and Mac will be able to see those colourful symbols if you use them in tweets. In contrast, the symbols in Feathers can be seen by people using any device or computer.