Custom Field for CiviCRM Membership not accessible from Drupal Rules module
There are a few things you can try here:

Go to CiviCRM's Administer menu &gt;&gt; System Settings &gt;&gt; CMS Database Integration.  Compare the array there against the one listed in settings.php.  This array (in settings.php) must be updated when you add new custom field groups to CiviCRM (since they add new tables).  This is most likely your issue.
Consider installing the civicrm_entity module, which should make your CiviCRM entities available as Drupal entities.
Consider using CiviRules, which is a CiviCRM-native rules engine.  It's very similar to Rules, but it generally understands CiviCRM better.