That square plastic trunking can normally be very easily opened - it just clips together. Turn the isolating switch off first. Inside the trunking will be the copper refrigerant lines (each insulated), the power cable between the inside and outside units, and the condensate drain tube. The condensate tube will either be leaking, or if it goes uphill and water can pool in it, it can get cold enough for condensation to form on the outside of the pipe, and start dripping too.
Thanks, I am going to get the installer back because it's likely not installed correctly or something has gone wrong. Mitsubishi say 5 years warranty on installation.
I think the guy reused almost everything from the previous installation of the Fujitsu we had before.