I actually got this working, so no problem. If anyone else wants to do this you can add this in your functions file…
function mpdf_meta_box(){
add_meta_box('generate-pdf-action', 'PDF', 'mpdf_meta_box_action', 'plan', 'side', 'low');
}
add_action('add_meta_boxes', 'mpdf_meta_box');
function mpdf_meta_box_action(){
if(function_exists('mpdf_pdfbutton')) mpdf_pdfbutton(true, 'Generate plan PDF');
}
I had one more query, how do you get the PDF to auto download?