Thanks for the solution, it really helped. However I'd like to make it a bit more accurate: the crucial step is number five: unzipping the cod. (The MIME types are probably needed as well, but they were installed by default on my debian.)
I'm just saying this to point out that the JDE build (I'm using NetBeans and ant to build my projects) and the signature doesn't have anything to do with the problem. It's a strange one though, as the JDE tools seem to create an install file that then cannot be installed on the BlackBerries.