My info is dated but I can't see much changing since then.
Mayo is a float plane landing spot. Not much other than that. Lots of room for setting up your tents and has outhouses. No canoes or canoe rentals unless something dramatic has changed.
Normally people rent from a Whitehorse expediter, have them drive people, gear and canoes to Mayo. They'll also contact Black Sheep for your flights to McClusky Lake and from Taco Bar back to Mayo. And, of course, getting you, your gear and their canoes back to Whitehorse.
FWIW, it is one of the best if not the best northern canoe trip I've ever taken. The scenery is stupendous and the hiking great.
The webpage of my trip has Lat/Long for McClusky, the confluence of the starting creek and the Wind, a couple of great campsites, Peel canyon which should be scouted first (no portage that I know of) but no big deal once scouted. I also have Taco Bar co-ords as you don't want to miss it - it's a long way to Fort McPherson and it can be easy to miss. The float plane sign if often on the ground.
We've used Up North Adventures
http://www.upnorth.yk.ca/ for several trips and really liked their attitudes, professionalism, prices and gear.
Enjoy - I know you'll love it.
Ted