Toulouse will face the winner of the second semi-final between Clermont and Toulon, which takes place on Sunday.
McAlister kicked six penalties, including two from inside his own half, to put the hosts on course for victory before Lionel Beauxis added another six points with the boot, including a drop goal.
“I’m really happy,” McAlister said.
“Everyone in the dressing room was. Tonight my kicking was good. I haven’t had that kind of success with the boot lately.”
The only time either side looked likely to cross the try-line was just before the interval when Toulouse were reduced to 13-men, Florian Fritz being shown a yellow card for a high tackle in the 32nd minute while Timoci Matanavou was sent to the bin five minutes later for cynical offside.
Max Evans took full advantage of the overlap to break clear of the Toulouse defence, only to be chased down by Vincent Clerc and rolled onto his back on the try-line.