I know you already found a solution, but I can't help but asking: Did you name the .dat as MsetForBoss.dat or loadcode4.dat/loadcode.dat? It should be named MsetForBoss.dat
I'm sorry but i do not understand what you are trying to say.
The only files need to install MSET are
1) code.bin (can be found on a link on the first post)
2) GW_INSTALLER.NDS (found in gateway's package)
3) Launcher.dat (for testing) (found in gateway's package)
put the above stuff in the root of the SD card and go to one of the above(previous post) mentioned sites on the 3DS.
bottom screen will flicker, and then wait for like 2~3 mins, and you will be forced back to the home menu, with an error message saying "an error has occurred, save all your work and restart".
Then test your MSET !!!