Hello,
We are sorry for late response.
I checked functionality in Joomla 3.1.1 and Joomla 2.5.14 and I can add new customer fields.
If I understand it right, you have no buttons under Extra-Felder. There should be button New to add your own fields.
Can you check logs, if you get any error? If you have no error, can you send us admin login details to your administration part of website? Please, send it to
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