Here's a snippet from one of my sites.
Code:
var fUploads = 0;
function addField()
{
document.getElementById('fields').innerHTML = document.getElementById('fields').innerHTML + "<input type=\"file\" name=\"uploads[" + fUploads + "]\" /><br />";
fUploads++;
}
Code:
<a href="javascript:addField();">Add Field</a>
You should be able to figure it out from there. If not, just post back.