Generating a random number in actionscript 3 is very similar to java.
To generate a random number betwen 0 and 100:
var number:Number = Math.round(Math.random() * 100);
Where Math.random() generates a random number between 0 and 1 and Math.round(), rounds the result to the nearest integer.
If you want to generate a number within a given range [start, end]:
var number:Number =Math.round( start + Math.random() * (end - start));