function findIndexInArray(value:Object, arr:Array):Number {
	for (var i:uint=0; i < arr.length; i++) {
		if (arr[i]==value) {
			return i;
		}
	}
	return NaN;
}

var myArray:Array = new Array("a","b","c","d","e","f","g");
trace(findIndexInArray("c", myArray));

// OUTPUT
// 2

If the item cannot be found then the return value is NaN. This function only returns the index of the first occurence found, so it's not very good if you have multiple occurrences of the same value in the array.

AS3 Search for a Value in Array and Return it's Position Index

number, search, array

ActionScript 3