Found it!
I did not know, you have to fill the complete list
Catalog Status: Enabled
Available In Admin: : Enabled
Sort Order: 0
Applies to product types: all
Directly Searchable: Yes
Show Field In Product Listings: No
META Keyword: No
Uses A List Of Values: Yes
User May Select: single value
Selected Using: Pull Down Menu
Restricts Product Listings: Yes
Show Parent Values: Yes
Quick Search Box: Yes
Number Of Check/Radio Columns: 2
Display Values As: Text Only
I got the same screen you got. Unfortunately I don't use this add-on. But I got a tip.
You can make a sub domain with the latest Oscommerce 2.3.1. Then you install this add-on on this version. And looks what's different in working process. Maybe something is missing in the database? I don't think there is a button missing.
You can find the button here:
adminsite -> extra_values.php 424
Code:
if (!$field_info['epf_multi_select']) echo tep_draw_form('sub', tep_href_link(FILENAME_EXTRA_VALUES, 'get')) . tep_draw_hidden_field('action', 'new') . tep_draw_hidden_field('list_id', $list_id). ' ' . BUTTON_SUBVALUE . ': ' . tep_draw_pull_down_menu('parent', tep_build_epf_pulldown($eid, $lid, array(array('id' => 0, 'text' => TEXT_NONE))), 0, 'onChange="this.form.submit();"') ; ?></td>
BUTTON_SUBVALUE => you can find the definition in the language file
define('BUTTON_SUBVALUE', 'Add A Sub-Value To This Value');
When I click the button the page reloads. That's because FILENAME_EXTRA_VALUES is the same page.
I update the file a little. So there is a new screen. The next problem is how to put the information into the osc_extra_field_values table.