hmm... the thing is atom-mocha does not rely on outside installs of mocha....
The problem that you're showing is not in any way related to atom-mocha the plugin....
Also there is no need to install mocha separately for this plugin to work...

But i think i can help with this anyway :P

npm install <package> --save-dev does not install packages globally, but locally within the current npm project (i.e. the current folder) as a dev time dependency.

Can you please share a screen shot of the error message you get so we can clarify this further??

I'm 100% sure that this is a problem with things other then atom or atom-mocha, but not sure what yet :P

Cheers,
Boogie

(also please note that i don't really maintain this package anymore, but i can help with issues like this :P)