1. Admin Customer Details: no extra fields show
Of course not. No content has been filled from the customers yet. It will only show up when customers will fill something inside each new create additional customers fields.

2. Customer/storefront: Login form shows box with the heading "Additional Customers Fields" but the added field does not show
Sounds like you did not went into your template files and languages files for adding the required initials of your additional customers fields once you activated the field from the admin - > customers - > add customers fields page.

3. Customer/storefront/my account info: Same heading but empty box as #2
Here you go. Perfect sense. The content has not been filled by the customer from your account edit page. Of course, for the registration page, you must at least follow my reply right above for your 2..

Also, from reading through this thread, I made sure more than one additional field was created.

Any ideas? If I'm missing any details to help with an answer, please let me know. This would be an AMAZING help if I could get it to work. I've just been working at it for awhile and have a feeling I'm overlooking something.
Sure. Apply my steps on this thread.

<?php // Additional customers fields begins here. ?>
<?php if (isset($account->fields['field_name'])) { ?>
<fieldset>
<?php if (isset($account->fields['field_status']) && $account->fields['field_status'] == 2) {
?>
<legend><?php echo ENTRY_ADDITIONAL_CUSTOMERS_FIELDS_TITLE; ?></legend>
<?php
} // End of if statement.
while (!$account->EOF) {

// We now set up an additional customer field for hobbies.
if (isset($account->fields['field_name']) && $account->fields['field_name'] == "customers_hobbies") {
echo add_customers_fields_for_create_account('customers_hobbies', 'zen_draw_textarea_field');
} // End of additional customers fields for hobbies.

// We now set up an additional customer field for sales representative.
if (isset($account->fields['field_name']) && $account->fields['field_name'] == "sales_rep") {
echo add_customers_fields_for_create_account('sales_rep', 'zen_draw_pull_down_menu');
} // End of additional customers fields for ages.

// We now set up an additional customer field for cellphone.
if (isset($account->fields['field_name']) && $account->fields['field_name'] == "customers_cellphone") {
echo add_customers_fields_for_create_account('customers_cellphone', 'zen_draw_input_field');
} // End of additional customers fields for cellphone.

// We now set up an additional customers fields for pods.
if (isset($account->fields['field_name']) && $account->fields['field_name'] == "customers_pod") {
echo add_customers_fields_for_create_account('customers_pod', 'zen_draw_radio_field');
} // End of additional customers fields for pods.

$account->MoveNext();
} // End of while statement.
?>
</fieldset>
<?php
} // End of if statement.
// End of additional customers fields.
?>
AGAIN ! - Please post your messages within 1-2 threads for your problems (for each problems at least). I'm sorry but the next time I'm seeing this - no more support for you !

That does look right ... hope I didn't coded my function wrong. I will test this in a couple of mins and confirm if this is a bug or not regarding the multiple fields showing up twice (or more).

Also if I go to edit a customer from the admin side my pull down menu shows up, but there is no content in the pull down box. So, then I also create a radio button selection and it shows up but still no pull down. What am I doing wrong? Here is my admin/includes/languages/english/extra_definitions/additional_customers_fields.php
$additional_customers_fields_form_pull_down_title['sales_rep'] = array('Sales Representative: ');
$additional_customers_fields_form_pull_down_content['sales_rep'] = array("None", "Mel Kimmel", "Boyd Thomas");
What did you selected for the field type ? I just noticed I forgot to state an important line in my documentation.

All drop down fields must be created as INT in order to see the pull down menu's content.

Sorry about that. I will add this part into my documentation immediately and leave an update when my package will be ready.

For now, you may delete your sales_rep field from the admin and re-create it again (as int this time). It should work from there - thanks for the catch.

Also, is there any way to get this additional field to show up on the invoice when I print it?
I'm not sure what you mean there ... all customers who completes an order should have the additional customers fields included (and this - of course - if you have followed my instructions regarding the email template and checkout_process.php's language file). If it wasn't what you meant, please advise.

I also get this error when I try to update the customer from the admin side.

1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE customers_id = 36' at line 1
in:
[UPDATE customers SET WHERE customers_id = 36]
If you were entering information, press the BACK button in your browser and re-check the information you had entered to be sure you left no blank fields.
Please read 5-6 thread above. The reason for this has been mentionned.