There is no airport in Mesolongi. The closest airport to Mesolongi is the airport of Patra-Araxos. This small airport works only in summer and receives few charter flights from abroad. From Patra, there are daily buses to Mesolongi.
Alternatively, you can arrive in the International Airport of Athens that receives many flights from all over the world. From Athens, you can go to Mesolongi by bus that departs from Kifissos Bus Station. The bus trip from Athens to Mesolongi is 4 hours.