Stuart means the small button with a right-pointing triangle (see attached image). You will see it if you open the Stair tool's settings dialog.
If there is a stair selected, and it is an object from the included library, the button's expansion will not let you edit it. It will just read "Create new stair" and if you select that, it will transfer you into Stairmaker, where it's possible to make a stair the way you describe it.
If you save this Stairmaker stair, and later select it, you will be able to edit it through this button as Stuart describes.
I think this way we start Starimaker is one of its worst problems, it's like if GS wants to hide it from all but the most daring users.
AC4.1-AC26SWE; MacOS13.5.1; MP5,1+MBP16,1