Here are the steps for get this work.
1.) Open the file "config.cfg" with "notebloc" (Sorry Idk what's the exactly name of this in Engish computers), in the end where says "Dissablehiddenfuctions=True" change it to "false".
2.) Open SMBX (you will see a message box here), close the game.
3.) Go to "level settings" and check out the button "advanced", if this button have a color (other than black) then go to the 4th step. If you don't see the colorful text, then close and open the game many times as needed, the idea is that you see something like this:
4.) Keep presing "shift" and now do click in "advanced", a message will appear saying "You found the secret function, but you need to beat a minigame..." and then, a mathematical game will appear:
5) Beat the game.
This secret was discovered by myself, I haven't beaten the minigame because it turns TOO hard, I don't know what's is the number of question you should beat, I just got to make 50 correct questions. If you make something wrong, your score will decrease.
What happen if you finish the minigame? What are the hidden functions? Who knows, Can you beat the game? Personally I got stressed to try...
I hope this info will helpful!
I wish you good luck! and help us to discover the game!