Path problems for IPB site

You don't need to specify the path to IPB. Drupal only cares where the data is stored so that he can check if the user/pass you provide are in the IPB database. Just make sure that your Drupal host can access MySQL on the IPB host.

In your setup (2 different hosts) you need to replace localhost for IPB with the IP address of the mysql server that is used by IPB (and you also need to make sure that the server is listening on that IP address). You will have something like this in your settings.php:

$databases = array (
  'default' =>
  array (
    'default' =>
    array (
      'driver' => 'mysql',
      'database' => 'main_database',
      'username' => 'drupal_database_user',
      'password' => 'drupal_database_pass',
      'host' => 'localhost', // or change localhost with something else if your Drupal database is not on this server
      'port' => '',
      'prefix' => '',
    ),
    'ipb' =>
    array (
      'driver' => 'mysql',
      'database' => 'ipb_database',
      'username' => 'ipb_database_user',
      'password' => 'ipb_database_pass',
      'host' => 'ipbhostname', // this is the IP/hostname of your IPB mysql server
      'port' => '',
      'prefix' => '',
    ),
  ),
);