Hey there. You can hire workers by selecting the settlement in the upper left, then clicking on the button that appears in the sidebar. That spends treasures though, so be careful.
I have the same issue. There are no buttons in the side bar when I've selected the settlement. I'm using the file from the runnable Love project download and Love 0.10.2 on windows 10.
Yeah, not all settlements have buttons, only training camps and rest camps. There should be one in the top left corner of the map. You just got to scroll the camera there by dragging it with the middle mosue button.