Up to 2009 you could access the tensioner from behind the right front tire. I don't think they changed this for 2010. Jack the car up, use a jack stand, and remove the tire. Remove the lower splash shield. Never did it this way so I can't say it's easier or better.
Since you've gained access to the tensioner from above you may as well continue from there.
I'm guessing you loosened the bolt that that goes through the arm into the engine or front cover. Tightening that bolt does not lock the tensioner into place, it simply holds the tensioner on the motor. The torque for the bolt is 33 ft/lbs. I don't know how much you loosened the bolt or pulled the tensioner away from the engine but if you loosened it a lot and moved the tensioner 1/2 inch or so away from the motor it is possible the locking dowel slipped out of it's hole. If that's the case then you will need to install the tensioner correctly and torque the mounting bolt to 33 ft/lbs.
The tensioner has a spring in it that keeps tension on the belt. The square hole is near the pulley end of the arm. If you put a 3/8 ratchet in the hole and rotate the tensioner counter clockwise the belt can be removed (theres no reason to loosen any bolts). You don't have to take the belt right off, just get it off at least one pulley. With the ratchet still connected to the arm let the tensioner move back clockwise. It will move clockwise until it hits the stop.
In either case the belt needs to be off the tensioner to correctly torque the bolt.
To put the belt back on, get it on as many pulleys as you can, then with the ratchet rotate the tensioner counterclockwise. Slip the belt onto the tensioner pulley or whichever one the belt isn't on and slowly let the tensioner go back clockwise.
This link is a picture of what the tension mount look like on the engine side.
Running the engine with the MAF dangling might throw an engine code but that should go away once everything is hooked up again.