Try changing your plugin to this:
PHP Code:
ob_start();
switch ($forum[forumid]) {
case "1":
$forumad = "this ad will show up in forum #1";
break;
case "2":
$forumad = "this ad will show up in forum #2";
break;
default:
$forumad = "this is my default ad";
break;
}
ob_end_clean();
Just make sure the hook location is postbit_display_start and then you'll put
$forumad wherever you want it to be within your postbit_legacy template.