karena kamu fetch data dari database kamu tidak bisa pakai array_sum, tapi dengan penjumlahan biasa seperti:

<?php
 $total = 0;
while($r=mysql_fetch_array($q))
{
    echo"<tr>";
	echo"<td>" . $no . "</td>";
	echo"<td width=15% >" .$r['kode_mk']."</td>";
    echo"<td width=100%>".$r['mk']."</td>";
    echo"<td><center>" .$r['sks']."</center></td>";
    echo"<td><center>" .$r['grade']."</center></td>";
    echo"<td><center>" .$r['NxB']."</center></td>";
    echo"</tr>";
    $total += $r['NxB']; // penjulan
	$no++;
    }
    echo"</table></br>";
    echo "TOTAL:".$total;

atau kamu bisa pake query select sum seperti:

<?php
$sql = "select sum(NxB) from nama_table";
$q = mysql_query($sql);
$row = mysql_fetch_row($q);
echo "TOTAL:".$row[0];

thank gan:) sngat membantu..