Error "function not found"

I have a not very easy installation on an Internationalized domain name (“ä” German). With the settings under dm_admin_page I get the following error message:

Warning: call_user_func_array () Expects parameter 1 to be a valid callback, function ‘dm_admin_page’ not found or invalid function name in /www/htdocs/xxx/wp-includes/plugin.php on line 406

What could that mean? Tried a lot, nothing helped.

Thanks for support.

http://wordpress.org/extend/plugins/wordpress-mu-domain-mapping/