the 3ds port is based off v2.1.20, so that means mods made for 2.2 won't work, and 2.1 mods that are pk3s also won't work (pk3 support was added in v2.1.21) and because you can't access the console or use the add-ons menu (it was added in v2.1.22) you have to use autoexec.cfg
simply make a file called "autoexec.cfg" in the same directory as your assets and open it in notepad then add:
"addfile *modname*.wad"
replace "*modname*" with the file name of your wad, so for example:
"addfile scr_mysticrealm.wad"
if the mod you have is a pk3 then you gotta open slade and make it a wad (which I will not explain)
and 2.2 mods will 100% not work properly