Can't download SimCity expansion pack (Cities of tomorrow).

First, make sure you have purchased the base game SimCity. The Cities of Tomorrow expansion pack is not a standalone game, it's an addon to the main game.

If you've purchased SimCity, then Cities of Tomorrow will just show up in the game when you log in; you don't have to download it separately. In Origin, click the info button on SimCity and you should see it listed under the expansions. In-game, the Academy and OmegaCo buttons should show up under the city specializations tab.