You can arrive directly in Nantes international Airport or through one of the 3 airports of Paris, CHARELES DE GAULLE,  ORLY or BEAUVAIS. It is thus easy to make a stopover in PARIS on the way to Nantes. 
 
Nantes’s airport is international. Some direct flights are operated from most of the largest European cities and from North America (New York, Montreal etc). Most of these flights are operated by low cost companies and you can find more information on the website of the airport (https://www.nantes.aeroport.fr/en/homepage) 
 
In case of arrival in Paris, two main airports can be considered either Charles De Gaulle (most common) or Orly. High speed “TGV” trains are operated every day from these airport either directly at Charles de Gaulle (~4 every day) or from MASSY train station from ORLY airport (ORLYVAL rapid shuttle from airport to train station). You may also take the metro (RER C) from Charles De Gaulle airport to downtown Paris and then take a train at MONTPARNASSE train station (metro station Montparnasse) where train leave to Nantes every 1 to 2 hours. It is thus easy to organize a stopover in Paris over your day of arrival or for a longer duration. 
More about train booking https://www.sncf-connect.com/ (June will be start if summer season, advanced booking is recommended) 
More about transfer in Paris and in Nantes; the application MOOVIT is very convenient (Android, ios)
 
A third alternative Airport is BEAUVAIS ( https://www.aeroportparisbeauvais.com/en/passengers/ ) which welcome only low cost companies. From BAUVAIS AIRPORT, buses are available to reach either Paris or Nantes (https://www.busbeauvaisparis.com/  or again https://www.omio.com/ ). The transfert from BEAUVAIS to PARIS takes around 1h15. 
 
In NANTES, the congress centre is at walking distance from the train station (15 min). Shuttle buses connect the airport to the train station every hour. Taxi can be used too from the airport (around 50€)  
Use the application MOOVIT to find the best solution for mass transport in Nantes.