The Robins, managed until February by Paolo di Canio, finished in sixth place, five points behind Uwe Rosler's Bees but they have not tasted defeat on home soil since February.
"The players are excited, they're quietly confident because that's the way they are," said MacDonald.
"Nobody knows how the game is going to pan out on Saturday. We've got a lot of ideas on how we'd like to play but I'm sure Brentford have got a lot of ideas on how to try to stop us playing.
"They're a very good side, obviously they just missed out on automatic promotion after 17 seconds of mayhem at their place last week.
"Uwe Rosler has a very good team there and they've been up there for most of the season as well.
"I think both teams are very evenly matched, I think all four teams in the play-offs are very evenly matched so it might be a flash of brilliance that changes the game or a decision by the officials - who knows?
"We're just hoping the decisions go right for us."