Hi there! The company that flies from Buenos Aires to Chapelco (the name of San Martin de los Andes' airport) is Aerolineas Argentinas (
www.aerolineas.com.ar). The are several car rental companies in the area, check this out:
http://www.interpatagonia.com/sanmar...ar-rental.html. I would recommend that you rent a car to drive in the Lake District area and take a plane or bus back to Buenos Aires, otherwise, the drop off charge of taking a cas in San Martin and return it in Buenos Aires will be really high.