Pick your desired square on the map and choose the found new village option. When an expansion slot is available for use, you can train settlers or a chief (if researched). Once you have used them (founded or conquered a vilage), the village will appear in the expansions. There is no special unlocking option for the slots.
All I am trying to do is make my village a city,
You may be thinking of the wrong game. Upgrading villages into cities is a feature in Travian: Kingdoms and is not present in Travian: Legends.