The new attributes are saved in GenericAttribute Table.
//save customer attributes
_genericAttributeService.SaveAttribute(customer, SystemCustomerAttributeNames.CustomCustomerAttributes, customerAttributesXml);
You can Get them using:
customer.GetAttribute<string>(SystemCustomerAttributeNames.CustomCustomerAttributes, _genericAttributeService);
But you receive an xml that you have to parse with
var selectedValues = _customerAttributeParser.ParseCustomerAttributeValues(selectedAttributesXml);