You answered your own question.
Actionscript and C2's event system in my experience is practically the same.
You would have a global var called Score,
Then add the event:
system.compare variable(Score > 100)
-> Star1.setVisible(Visible)
system.compare variable(Score > 200)
-> Star1.setVisible(Visible)
-> Star2.setVisible(Visible)
system.compare variable(Score > 100)
-> Star1.setVisible(Visible)
-> Star2.setVisible(Visible)
-> Star3.setVisible(Visible)
Something like that should get you going <img src="smileys/smiley1.gif" border="0" align="middle" />