The tank and fairing are designed so that by pulling the "C" clips at the rear of the tank with the seat up you can slide the tank back far enough to get to the coolant filler.
You will need to remove the battery covers first, and it's a good idea to put a towel between the seat and the tank when it is back to protect the paint on the tank.
Is the coolant reservoir empty? You may be able to put coolant in it that will be pulled into the system the next time a hot engine cools down. Fill it to the maximum mark, let the engine warm up to operating temperature, and then shut the engine down. Check the tank after the engine cools to see if any coolant was pulled into the engine.