Before use the easter egg, make sure that the OS version of your iPhone/iPod Touch's is at least 2.2, I have tested and the easter egg works at least on 2.2 and 2.21.

1. Launch Spell Number, and enter secret digit 9876543.21 91929394.59
   まずアプリを立ち上げ、9876543.21 91929394.59と打ちます。
   
   

2. Quit Spell Number by pressing the home button and return to home screen. Now open "Settings"
   そのままアプリを終了し、設定から
   
   
3. Go to General->Keyboard->International Keyboards->Japanese, and now you will find a new switch called "Emoji", check it if you want to enable Emoji keyboard. You can also uncheck it to disable Emoji keyboard later.
   一般->キーボード->各国のキーボード->日本語->絵文字が追加されてますので「オン」。これで設定完了です。
   
   
   
   
   
   
4. If you cannot find the emoji option, please reboot your iPhone/iPod Touch. We find that in newer iPhone OS, the preference is cached so we have to reboot the iPhone/iPod Touch to refresh the preference. You should able to see the emoji option after reboot.
5. Now the Emoji keyboard is enabled, go to any program (I use Notes for example), click any field to popup the keyboard, click the "Earth" icon until Emoji popup
   絵文字を使う時は、文章作成時に地球のマークをタップしてください。
   
   
   

You can also delete Spell Number if you don't want it, the setting will still be here.