Peep the example at the bottom of the page you linked to. I've pasted it here:
$result = $MailChimp->call('lists/subscribe', array(
'id' => 'b1234346',
'email' => array('email'=>'davy@example.com'),
'merge_vars' => array('FNAME'=>'Davy', 'LNAME'=>'Jones'),
'double_optin' => false,
'update_existing' => true,
'replace_interests' => false,
'send_welcome' => false,
));
print_r($result);
Your merge vars (ex. FNAME and LNAME) need to be in its own array. So, add a 'merge_vars' in your array and create an array that contains your field's merge variables.