By default, Commerce Auction (http://drupal.org/project/commerce_auction) puts the bid form on a separate page/tab from the auction node. The code below can be added to your theme's template.php file, and updated [minimally] to work with your site.

gistfile1.php

<?php
// 1. put your theme name below
function YOURTHEMENAME_preprocess_node(&$variables) {
  // 2. put your content type below
  if ($variables['type'] == 'auction_listing') {
    if ($variables['auction_timeout'][0]['value'] > time()) {
      if (user_access('place bids', $variables['user'])) {
        // display the bid form
        $node = $variables['node'];
        module_load_include('inc', 'commerce_auction', 'includes/commerce_auction.pages');
        $bidform = drupal_get_form('commerce_auction_place_bid_form', $node);
        unset($bidform['warnings']);
        $variables['content']['auction_bid_form'] = $bidform;
        $variables['content']['auction_bid_form']['#weight'] = 6;
      } else {
        $variables['content']['auction_bid_form'] = array(
          '#markup' => "<div class='loginmsg'>Please <a href='/user'>Log in</a> or <a href='/user/register'>Register</a> to place your bid.</div>",
          '#weight' => 6,
        );
      }
    }
  }
}