When searching Dutch surnames prepositions like 'van', 'de', etc. should either not be entered or be entered in a separate item called 'tussenvoegsel', 'prefix' or something like that. All indexes are sorted on the 'bare' surnames, so 'van den Berg' will be found at 'B', not at 'V'.
Note that the advanced search on wiewaswie is only available with a paid subscription. The basic search is available for free, but that is very basic indeed...