McAllister, who experienced Wembley heartache as Bristol City skipper under Gary Johnson in 2007/08, has helped his side to home and away wins over Uwe Rosler's Brentford this season.
And the 35-year-old believes the Glovers need to treat the final like a trip to Griffin Park if they are to clinch a hat-trick of victories over the Bees.
"What we are trying to do as a squad is to treat it like another game in our preparations," he told Press Association Sport. "We've beaten them twice this season but it is about who produces on the day and who handles the situation and the occasion the best. You've got to head in relaxed and look to play your game.
"It is a great day and a great occasion to get there only if you go and win it. I've been there and lost and it is a horrible feeling. For me it is just another game."
Bees striker Clayton Donaldson is hoping it will be third time lucky as they look to finally record a victory over the Glovers and attempt to make up for missing out on automatic promotion on the final day.
"It's not going to be easy," he said. "We've lost twice against them this year - it's not going to be a walkover and it'll be a great game.
"There's still a lot of work to be done. Getting to the final is one thing, winning is another. We've got to keep our feet on the ground. It wouldn't count for anything if we lost it. We've still got a job to do.
"I've never played at Wembley. It's always been my dream to play there, I always hoped it would be me one day.
"On paper we're the third best team in the league and we deserve the chance at Wembley."