Grease 13x9-inch pan with shortening or cooking spray. In large bowl, mix cereal and trail mix; set aside.
In 2-quart saucepan, melt butter over medium heat. Stir in brown sugar, flour and corn syrup. Cook, stirring occasionally, until mixture comes to a full boil. Boil 1 minute, stirring constantly.
Pour mixture evenly over cereal mixture; toss to coat. Press mixture in pan. Cool completely, about 30 minutes. Cut into 6 rows by 6 rows.