Help with basic hook for my first module
dpm and dsm can be a little tricky in some hooks, and you may need to time a cache clear with a page reload to see them fire.

Better to use something like watchdog or actually alter the page content.

Your function looks good, so try this from the example module. More chance of seeing the effect:

function checkout_pane_example_commerce_checkout_pane_info_alter(&amp;$panes) {
  if (!empty($panes['cart_contents'])) {
    // Change the title of the "Cart Contents" pane.
    $panes['cart_contents']['title'] = t('Your big wonderful shopping cart brimming over with shopaholic nonsense');
  }
}