You could take the 747 airport bus to downtown Montreal and then take the Galland bus to Ste Agathe. The Galland website is never correct so you do need to call ahead. But this option does involve a walk of at least 20 mins to the lake site.
Where are you staying, as that makes a difference on the walk you might have.
To be totally honest I would rent a car and drive the 75 mins it takes, the bus option is long, not that cheap and once in Ste Agathe you will not find getting about easy without a car. It is a small town with no transit system.
Anything else you need to know just ask, I am there a few times a week.Edited: 10 years ago
Yes not transit in town and it is not always evident to get around. Also, hopefully, you are not whishing to get back to Montreal the same night as this will be impossible unless you rent a car. Beside the Galland buses there is also the Transport Intermunicipal des Laurentides that cost less hand has more departures but it is quite convoluted. If interested to know about this second option then just let us know.
U def need a car...ste-agathe-des-monts is about 1hr 20min driving distance (highway time at 115km/hr) from Dorval airport which is named "Montreal-Trudeau Airport" . There are many rental car dealers on site at the airport....here are a few it may help u.
Alamo Location D'Autos, Montreal - Aeroport P.E.Trudeau, Montreal - (514) 633-1222
National Location D'Autos, Montreal - Aeroport P.E.Trudeau, Montreal - (514) 636-9030
Budget Rent-A-Car Montreal - Aeroport P.E.Trudeau, Montreal
Avis Rent-A-Car Montreal - Aeroport P.E.Trudeau, Montreal
gooooooood luck ...see u at the show..:-)