In Run.GPS, go to Configuration >> Sensors and set the type to "Bluetooth Low Energy". Then search for a heart rate monitor device (which must be worn at this time to be switched on). You should be able to detect and connect the device now.
Well the Polar M400 uses a standard Bluetooth Smart (4.0) heart rate monitor strap, so it should be compatible with Run.GPS (Android, Blackberry and iOS, but not Windows Phone). But we did not test it yet ...