So a lot of times I come across the problem that I have to look up a word or check the detailed meaning of a word when reading and writing dutch. 

Most of the time until now I used van Dale widget, which gives you a good solution but I never found a good way to integrate with the existing dictionary in OS X. So now solution found.

On the the website http://mysite.mweb.co.za/residents/clasqm/os-x/extra-dictionaries-for-dict/dutch.html

I found several dictionaries as

From Dutch:

Čeština ➠➠ Nederlands (2.8 MB)

Dansk ➠➠ Nederlands (442KB)

English ➠➠ Nederlands (1.6 MB)

English ➠➠ Nederlands (1.6 MB)

Français ➠➠ Nederlands (2 MB)

Svenska ➠➠ Nederlands (1.3 MB)

To Dutch:

Nederlands ➠➠ Čeština (2.5 MB)

Nederlands ➠➠ Dansk (726KB)

Nederlands ➠➠ Deutsch (3.5 MB)

Nederlands ➠➠ English (4.6 MB)

Nederlands ➠➠ English (2.2 MB)

Nederlands ➠➠ Français (3.3 MB)

Nederlands ➠➠ Svenska (1.4 MB)

that you just can download and add to your system folder for dictionaries, after restarting Dictionary.app in OS X you can use these ;-).