Taxis operate throughout the city (watch the front window for max 1.00 Lev/km in day hours; from Sofia International Airport Terminal 2 to the Symmetry Festival venue it should cost you ~10 Levs or 5 EUR; if you use taxis at the airport, inside the airport turn to the right exit, where there is an official taxi stop).
From the Sofia International Airport metro station (turn to the left follow the blue line and go to the end in the airport hall and then go in front the airport) to the festival venue it should cost you only 1.60 Leva (=0.8 EUR): ~50-60 minutes travel − you must transfer from the red metro line at the central Serdica Station to the blue metro line for two stops in direction Vitosha, to station European Union which is just in front of the National Museum “Earth and Man”.
At the airport there is also bus N84 or N284 to downtown (tickets can be bought at the kiosk outside the airport or from the driver; 1.60 Lev). If you arrive at Sofia International Airport Terminal 1 (low cost flights), you can take a bus to Terminal 2 to the metro station (~5 minutes travel), or you can take a bus or a taxi downtown. If you arrive by train or by bus at Sofia Central Railway Station or at the near-by International Bus Station, you can find your way to the metro station and you are can travel 4 stops on the blue metro line to the European Union station without any transfer (~15 minutes travel; also 1.60 Lev).