<p>How important are they in an app?</p>
<p>success in math competitions is one way to show ability and interest in math/CS. Other ways are via advanced classes, research, summer programs. </p>
<p>(btw, a much better competition than ACSL is <a href="http://www.usaco.org%5B/url%5D">www.usaco.org</a>)</p>
<p>I think the mere fact that you participated in them is more important than the score. A month ago, ppl who were accepted gave their AIME scores and there were a surprising number of ppl who got between 1 and 3.</p>
<p>Not having those activities is not a barrier to admission, nor is having them a guarantee of admission. But they are appropriately math and science focused activities for young people who like to go beyond the school curriculum. If you enjoy them, they are great.</p>
<p>If your score is high, it obviously displays either a strong natural gift or incredible passion and devotion, both of which are a part of the "fit" for MIT. That being said, it's just one of many ways to display a strong interest in science and mathematics, not intrinsically better than any other activity.</p>