What's the error message that you're getting?
I also noticed that you're not doing anything with the 'col' value that you're sending...
If you want to sort them based on the column title that they click on, then you may want to add this bit near the top:
.......
$database="cs_jobs";
$col = (isset($_GET['col']) ? $_GET['col'] : 'jobnum');
mysql_connect(localhost,$username,$password);
@mysql_select_db($database) or die( "Unable to select database");
$query="SELECT * FROM jobs ORDER BY $col DESC";
$result=mysql_query($query);
$num=mysql_numrows($result);
.......
Notice the change I made in your ORDER BY...