The Modulus is the remainder of division between two integers. I was looking for this operator in Flash a little earlier. Flash CS3's help files are fairly difficult to wade through at the best of times and simply typing 'Modulus' into the search box didn't bring up any search results in Flash's help files.
I hit Google and discovered that the operator required is the % symbol:
X % Y = the remainder of X / Y
For example:
10 % 3 = 1
10/3 = (3*3) + 1(remainder)