@askbond, To get new farmers (The units you deploy on buildings such as Farms, Market, Barracks) You can only get them on the events during the D-Day (The months with the skull on them). It's quite by chance and luck, so that game really will test your lucks. To heal you need to end your turn while the farmers are inside the Shelter :)
@jaonewguy, Thanks for the feedback!
@t-c, For the farmers to appear on the structures such as market, farm, and barracks, you need to deploy them. You can see the farmers inside the shelter, click on them and then a window will appear as to where you will need to deploy them. :), and to sell wheat in the market, you need a farmer deployed in the market then input the amount you want to sell below the window when the market is clicked, then end turn to gain the profit for selling the wheat :D