This is the README for the NSMBDS MvsL "More Levels" patch. 

To edit the new levels:

Apply the .xdelta patch to a NSMBDS ROM, specifically "0434 - New Super Mario Bros. (U)(Psyfer).nds".

Then, go to the "Unnamed Files":

File 2089: MvL01_bgdat.bin
File 2098: MvL06.bin (really MvL01.bin)
File 2090: MvL02.bin
File 2091: MvL02_bgdat.bin
File 2092: MvL03.bin
File 2093: MvL03_bgdat.bin
File 2094: MvL04.bin
File 2095: MvL04_bgdat.bin
File 2096: MvL05.bin
File 2097: MvL05_bgdat.bin

and export the level you'd like to edit. (MvL01 being first custom MvsL level ect) Then, go to the 'course' folder in the ROM's 'root' directory and replace one of the areas. For example A01_1.bin (and A01_1_bgdat.bin), is 1-1 Area 1. Import your MvL custom level into the respective files, and then edit that area in NSMBe's level editor. Then, export the area you edited from 'course' and import back into the MvsL custom level you want to replace, and play in-game.

To load the new levels, you'll want to press SELECT while choosing your MvsL settings & course. If you hear the 1-UP sound play, the new levels are loaded. However, if you hear the cancel sound, it means the original levels are loaded, essentially allowing for 10 MvsL courses!

In the future, there'll be a NSMBe mod which'll add these levels to the MvsL section in the level listing, allowing for easier editing. 