If you set the name of the select list using brackets like: name="ProdCatID[]"
Then multi-selected values will automatically be submitted as an array. That way you can simply apply the insert for the CatProduct table and then manually add a FOR loop around it like:
<?php
if (isset($_POST['ProdCatID'])) {
for ($x=0; $x<sizeof($_POST['ProdCatID']); $x++) {
?>
[YOUR INSERT CODE HERE]
<?php
}
}
?>
When you bind the column in the insert server behavior, set it to:
<?php echo($_POST['ProdCatID'][$x]); ?>