Here is a very good example..
http://datatables.net/examples/server_side/custom_vars.html
Here is how you should initialise, my data and value will be your drop down value and your ajax method in controller will give the right data back
$(document).ready(function() {
$('#example').dataTable( {
"bProcessing": true,
"bServerSide": true,
"sAjaxSource": "../examples_support/server_processing.php",
"fnServerData": function ( sSource, aoData, fnCallback ) {
aoData.push( { "name": "more_data", "value": "my_value" } );
$.getJSON( sSource, aoData, function (json) {
fnCallback(json)
} );
}
} );
} );