how to configure whmcs smarty template plugin?-Collection of common programming errors

I have added the function.bnr_exchange.php to the includes/smarty/plugins directory. It contains smarty_function_bnr_exchange which returns a string.

I also modified invoicepdf.tpl

$pdf->Cell(0,6,{bnr_exchange} . '',0,1,'L','1');

which gives error: unexpected { I tried with “{bnr_exchange}” but then it writes the text {bnr_exchange}

If i simply write {bnr_exchange} in the template it says unexpected } in that line. So it doesn’t recognize the plugin “bnr_exchange”. My question is how do I tell whmcs that I have a file there with that plugin so it can see it?