First post here since moving to Linux so hello.

I am having trouble changing the application menu. I right-click the "start button" (is that correct Linux terminology?!) and then "edit applications". The KDE Menu Editor which opens already shows some menus which aren't on the acutal menu ("Development") for example. Certain changes do not seem to work (even when I click save). For instance, I cannot create a new menu group or add items to the first level. It DOES however let me edit existing items and change the order (move up/down).

I must be doing something stupid for something so simple, but have not found a solution despite searching... can anyone please help?

It'sa bit more involved than that -see "https://specifications.freedesktop.org/menu-spec/latest/".

The desktop menu works in a preset format, thus all the other hidden menus you saw, and AFAIK you can only work within this.