upgraded to 3.3.8, problem with Users Management

[Didier9]

I just upgraded my backup site to 3.3.8 (I do have my main site still on 3.3.4 for now until I know the bugs are worked out)

I copied the user_management plugin but it does not load the User Management tab from the Settings menu.
I tried to force loading the plugin with 

http://my_web_site.com/admin/load.php?id=user-managment (since that is what the tab is calling on the other site with 3.3.4)

and the page gave that error:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function '' not found or invalid function name in /xxxxxxxxxxxx/admin/load.php on line 39

line 39 is this:

call_user_func_array($plugin_info[$plugin_id]['load_data'],array());

Any help appreciated...

Didier