Installing the demo-version is simply unpacking the zip archieve. Starting is simply running the exe file in main directory.
Before every mission You can buy new units for in-game currency in "Store", choose which units You will use on level in "Stock" and press "Start" button. Then You must plan Your defense with chosen units by drag and drop them with mouse and don't let enemies pass. On the level You have to pay for each unit placed. Money drops from defeated enemies. You can also remove already placed units by "hammer" button in right bottom of the screen and get half of their price back.
Minigames use keyboard WASD buttons. In minigame with the Fatman catching burgers You can move him left or right by pressing A or D button respectively to avoid falling boots and catch falling burgers. In minigame with van riding a road You can move the van up or down by pressing W and S buttons respectively to avoid obstacles and reach destination point.
You can call in-game-menu by pressing the Esc button or by pressing "wrench" button in left bottom of the screen.