The Eutelsat Balloon is in the Parc André Citroën, a few blocks from the Balard station on the Paris Métro. (Take the Métro in the direction of Balard, which is at the end of Line 8.) Signs will direct you to the park.
If you're coming from the Eiffel Tower, the Musée d'Orsay, St-Michel, or other neighborhoods on the Seine, take RER Line C along the Left Bank toward Versailles Rive Gauche or St-Quentin-en Yveline and get off at the Boulevard Victor RER station. The park is hard to miss--just look for the balloon.
The balloon normally offers 10-minute rides from 9 a.m. until 30 minutes before the park closes, but the schedule sometimes changes, and rides may be canceled for a day or two at a time because of windy weather. (For aerodynamic reasons, the balloon's capacity drops as the wind increases, and rides are canceled when anticipated winds or gusts exceed 35 km/h or about 20 mph.)
For up-to-the-minute visitor information, check
www.aeroparis.com or call 01 44 26 20 00.