How do I create and add MP3-files to the albums?

Ciao again trumpetrolf. What do you mean with "creating" ? You can add MP3 files in CATraxx, but NOT creating them.
Am I maybe misunderstanding something ?

Adding MP3 Files: Manually
Open the track you need, then click on the Multimedia Tab and Open the little Yellow Folder in the Audio section.
Locate the file you need and confirm. Done !
From now on you will see a "Blue Note" ( instead than Black) in the Tracks Window and a "little loudspeaker" immediately after the Song Title in the Explorer.

Adding MP3 Files: Automatically
Double click on any track title in the Tracks Window, then MP3 > Attach MP3 Files
Locate the folder where you have your MP3's to add ( I strongly suggest to use one Folder per Album, if you use it often..), double-click on the Folder's name and click OK.
CATraxx will add them automatically ( matching Artist / Titles ) and when finished will open a message window telling you HOW MANY files where attached. Done !
The first times that you do it, you should check if everything is correct.
Here it works fine and I never had problems, if not for my faults...

ALSO: In the Help file, locate Using CATraxx > MP3 and you will find useful information

Hope this helps & have a nice weekend.