Chances are that this icon will be grayed out, which is why it’s easy to overlook.Click on any villager (worker) and it will be the bottom icon under Age IV buildings. At Age IV, the Wonder becomes available for building.

Each landmark advances to the next age (I > II > III > IV), so keep doing this until you reach Age IV, which is the final age.

You do this by clicking on a villager (worker), then select the 5th icon in the 1st row on the bottom left window.