How can I join an online Minecraft game on my iPad?
You Have to Jailbreak your device currently but in the next update will include online multi-player
You need to download the app first.
for Minecraft Pocket Edition, you can just play locally with other MPE players over Wifi. connect both devices to same network., Then start a world on the one of devices, then it will be showed on the other devices multiplayer menu.
Then just connect to the server.
From what I can tell, you can only connect to special Minecraft Pocket Edition servers: http://www.minecraftforum.net/topic/1256915-legit-minecraftpe-online-multiplayer/