>> But i cant use it in the designer, when trying to use it by drag and drop, it justs pops back to the palette. << sorry I have not seen this behaviour yet - what are the Guvnor and Designer versions you are using? Can you watch the designer part of the following video and let me know if you can reproduce the same - http://vimeo.com/29457538?
>> Next question is the usage of my work item java implementation, i havent heard a word about it in the video, from where comes the email work item java implementation? For new users it just looks a little bit like magic. << Good point. You can actually reference the documentation chapter 13 to get an idea on what to do - http://docs.jboss.org/jbpm/v5.1/userguide/ch13.html. The jBPM Web Designer does not deal with the actual workitem handler implementation..that is for the user to configure in their runtime.
>> Im doing a evaluation for open source process engines and i have to say that i am frustrated about the documentation and the "how to use" of JBPM. << We are constantly working on updates to the documentation, however most users will be able to see all that work with the jBPM 5.2 release docs (unless you build the jbpm-docs module from master now ;) ). You are always more than welcome to ask questions here or on the mailing list or irc as well.
Hope this helps.