Make a variable "score"
Make an event. Give that event a system condition of "every x seconds." Set the amount of seconds will pass to the amount of time between adding to score. Give the event a system action "Add To Variable". Make it add to "score" and set the number it adds to how much you want it to go up by every time the event plays.
Whatever you set the time to for every x seconds, is how often it will add to score. Whatever you set the add to score value to will be how much it increases.
For instance, every second add 1 to score would make it add 1 to score each second that passes.
Here is an example of what I am talking about:http://dl.dropbox.com/u/30187917/plaatforming_with_score.capxGreg2012-05-20 10:03:36