Hi Herbertie, welcome to MobileRead!
Installing a dictionary is easy - just copy it into the directory "ebooks" on your Gen3.
More problems you will have to find an English-Dutch dictionary. You can try your Dutch ebook stores if they sell dictionaries in mobi file format, you can try
www.mobipocket.com or Amazon. With Amazon you would have the problem that you have to strip the copy protection "DRM".
If all this doesn't help maybe you can repeat your question in the "Which one should I buy"-section, I think more Dutchs are reading this section than the Cybook part here.