The Mix receives control messages wirelessly from the instruments and produces sound output. It runs on a separate device that can be connected to quality loudspeakers. Using your phone or tablet, you can connect to the device and open a GUI to control the Mix.
Typically, the Mix will run on a Raspberry Pi. The Pi will create a WiFi hotspot to which instruments and devices can connect, and set its own IP so that it is easily reachable. Once inside the Pi's network, you can open the GUI by connecting to its IP on a specific port. Details on the deployment of the project on the Pi can be found in the DEPLOYMENT.md.
You can also run the Mix on your own machine. Be aware that you might need to modify the instruments such that they connect to your machine instead. Setup instructions can be found in DEVELOPMENT.md.
A final alternative is to use the version hosted at http://mcp.miguel-franken.com/. Again, you would need to ensure that instruments connect to this instance. But it might be the easiest option to simply try out the local instruments.