I can't edit a level from M vs L.
I opend a clean (unedited) ROM in NSMBe and Enabled DLP-friendly mode in the Tools tab and then I've edit a level, I saved it and when I played the level noting is changed .
and after that i open the level in NSMBe and i look to my level and everything wath i've edited is still there

Read the tutorial, you're missing something.

MvsL hacking guide

I will not tell you what it is so you'll read the tutorial.