It won't work, because the HTML it would render would be
<td valign='top' class='maintab_back'><a href='http://www.geocities.com/bloodepotter/MEDIA_PLAYER.html target='_blank''>M&A MEDIA PLAYER</a></td>
Notice the problem? Try
<script>
function addlinks(){
//Created by Agent Moose (smcodes.smfforfree3.com)
$(function() {
var links = new Array()
links[links.length] = ["M&A MEDIA PLAYER","http://www.geocities.com/bloodepotter/MEDIA_PLAYER.html' target='_blank"]
for(m=0;m<links.length;m++){
$(".maintab_last").before("<td valign='top' class='maintab_back'><a href='"+links[m][1]+"'>"+links[m][0]+"</a></td>");
};});}
addlinks();
</script>