How do I play Anno 2070 offline?

You don't.

From the Steam page:

Internet Connection: Temporary broadband connection for product registration, permanent broadband connection for multiplayer.

Translation: You must register online even if you only want to play it offline.

And it gets worse -- if try to play offline after registering online it'll still cripple the game for no apparent reason other than to punish you for daring to want to play the game without an internet connection.

This isn't even the worst customer-hostile DRM decision Ubisoft has made recently, but it's a good example of why I stopped buying Ubisoft games after Assassin's Creed 1.