Make sure your iPhone or iPad is connected to the same network as the running XPlane machine. The app will automatically find XPlane (through UDP) and will start receiving data and display it in the map. From the menu you will see some basic flight info (speeds, altitudes, etc) and you have the option to pause the simulation, or to follow the plane. Also from the menu you have the possibility to link and download your latest flightplan from Simbrief, you just need to fill your pilotID and download it. This will display extra flight information (arrival and destination airpots, distance traveled, ETA, etc). And also will display the flight plan waypoints on the map.
— I can't connect with X-Plane
Make sure that both the computer (where your XPlane simulator is running) and your iOS device are connected in the same network. If everything seems correct, it can be that your firewall is disabling the UDP communication, since it's a unsolicited traffic. Make sure UDP is not blocked in none of your ends (simulator and device) and try again. In case the issue persists, when the app is launched for the first time, a popup appears asking or permissions to "find and connect devices on your local network". It is necessary to give permissions otherwise the app won't be able to find XPlane. In order to give permissions, you can go to the settings of your iOS device, find the X-Plane Map app and enable Local Network. Otherwise, an easy way to see the permissions popup, uninstall the app and install it again.
— I'm not able to download the Flight Plan from Simbrief
The app always tries to download the last active flight plan. Make sure that the flight plan was "generated" from Simbrief, just "created" is not enough. Make sure also that the pilotID is correctly added in the app.
— The previous flight plan still appears in the map
As of now, the app only fetches the last generated flight. And keeps it (displayed in the Map and it's data in the Menu) until you manually dowload the plan again from the Menu.