CSS lightbox opacity IE7 not working

When visitor will arrive on website for first time, they will see a lightbox like
http://www.huddletogether.com/projects/lightbox2/
who show an help webpage and not only picture.

It works on FF, IE 6, Safari but not IE7, on IE7 opacity become 100%, the background is plain color and hide then website behind. I found some article speaking about:
filter: progid:DXImageTransform.Microsoft.AlphaImageLoader but it didn't works for me.

Here is my current css. Any idea on how to include the filter: progid:DXImageTransform.Microsoft.AlphaImageLoader to make this code working on IE7?

#overlay{
    position: absolute;
    top: 0px;
    left: 0px;
    z-index: 90;
    width: 100%;
    height: 100%;

      background-color:#333333;
      -moz-opacity:.50;
      opacity:.50;
}

* html #overlay{
      background-color:#333333;
      filter:alpha(opacity=50);
}