I don't have a button to add another diagramm to the model

Dear Tat,

Please tell us which Modeler version are you using and also please attach a screenshot of the point from where you are trying to create folders and add diagrams.

Below is attached an image of the Modeler v 3.3.2.0333 where you can perform the previous actions:

First, click on the diagrams Tab circled in orange. The blue arrow points to the New diagram button and the green arrow points to the Create folder button.

If this does not solve your doubt, please give us more detail on what's going on with your issue.

Best regards.