I found out what is causing the fact that nothing is imported. If you have a set of games, and one of those games has no .EXE or .COM file in the game folder, the entire set will not be imported.
I filed a bug report.
You can woraround this problem by deleting the games from the import in the last step of the dos importer wizard. (by using the DEL- key on the row you want to delete)