Edit a mesh from a mod

[Richydreigon]

So i got permission from a modder to edit one of his models, he made a mask i really like, but i would like to add a ponytail to it so it looks like a girl :tongue:
I dont know a lot about modding, so i thought i could just open his rar file, extract an obj file or something like that and just edit, but i think it aint that simple
so i followed this video

i do something like this (see image)
end up with my .obj file, but when i import it to blender nothing shows up
If anyway knows what am i doing wrong, or if there's a better way to do what i want please help!