Press the Green Flag to start the simulator. Use this EcoDisc like a real one! Most of the features are replicated to an extent, including the voices, and even the inspection controls! Click on small areas (like the call button and fixtures) to enlarge them on the left. You can then interact with them in this mode. If you manage to find keys and click on them (hint: they're on different levels), you can use them! Press I to open your Inventory to select them, then press U to use them in the place you're in.
