Dave,
Without understanding you multi-select code, I cannot be positive that what I suggest will work.
But, if your multi-select is actually creating an array - meaning it is saving the entire sting of ID is one variable, than you can just loop through each item. You could do the same if it is a list.
Let's say your variable containing the array is $courses,
You could then create a for each loop which will query for the related info and then display it.
<?php
foreach ($courses AS $item) { ?>
<?php
mysql_select_db($database_YourDB, $YourDB);
$query_course = "SELECT coursename, relatedinfo FROM courses WHERE id = '$item'";
$course = mysql_query($query_course, $YourDB) or die(mysql_error());
$row_course = mysql_fetch_assoc($course);
$totalRows_course = mysql_num_rows($course);
?>
<p>Course: <?php echo $row_course['coursename']; ?></p>
<p>Related Info: <?php echo $row_course['relatedinfo']; ?></p>
<hr />
<?php
mysql_free_result($course);
?>
<?php } ?>