That sounds like you're hitting up against the second issue with these cores that I mentioned. Ever since support was added for games to be loaded automatically on startup, these cores haven't worked standalone because they try to run a non-existent game. aliaspider is working on this, but in the meantime the recommendation is to run them via shortcuts instead, specifying a (correct) ROM for the core to run instead of the erroneous one it's receiving currently.
I described the process for getting this working in the second half of the post where I replied to you, after the edit.
Thanks you. I cant edit correctly the xml.
I want to add this:
<shortcut>
<executable>/3ds/mame2000_libretro/mame2000_libretro.3dsx</executable>
<icon>/3ds/mame2000_libretro/mame2000_libretro.smdh</icon>
<arg>/ROMS/ROMS mame/snowbros.zip</arg>
</shortcut>
But it result in a corrupt xml or something. Can you make the xml for me, please?