To hire Managers, please tap on the empty areas in your mine shafts, where you don't see human figures. After that, tap on "Hire manager" button on the menu that is shown.
After hiring Managers, make sure to activate them by tapping on the pickaxe and shovel symbol on the bottom right corner of your screen and then tap on "Activate".
After hiring Managers, make sure to activate them by tapping on the pickaxe and shovel symbol on the bottom right corner of your screen and then tap on "Activate".