I've got the same problem with the .dat file, only I use the Opus Reader and it doesn't have a System folder (I don't see it in Explorer), so I've tried to spread it out all over the existing folders, but no luck. It still doesn't activate.
On the other hand I didn't delete any books, because I'm not sure what to delete.., does it say that I should delete all the preinstalled books that came with the Bookeen Opus..? (That would be wierd!)
I hope you people can help, because half of the idea of buying the Opus would be lost, if I can't use librarybooks on it