Have you tried doing what BatVink just suggested?
Quote: "If anybody could tell me please which of the DBPro software I have to use and in what order it has to be installed, I would be very thankfull."
What you seem to have done should work. You just need to make sure when you install the upgrade that it goes to the right folder. For example, on my laptop the installer always defaults to the wrong folder because the first time I installed on that machine I installed a different version of DBPro in a special folder for testing purposes. If I don't pay attention when I install an upgrade then the wrong copy is upgraded and confusion follows till I realise what I've done.
One day I'll get it sorted out. One day ...
I suggest you try again and let us know, step by step, what you did. Don't forget to actually run the installer after unzipping it ... (it's been known

).
Edit You should still have the U7.4 installer on your machine somewhere. Just find it and run it again.
2nd Edit Just re-read your first post. I don't think you need the IDE updates - the latest version comes with the main upgrade (that's true for U7.5beta10 anyway - and I'd suggest you use that as the remaining problems with the editor are very minor really).