This is for Developing Packages.

Guide for windows

Open up atom. Navigate File > Settings. Click on the little open Atom button:

Once open navigate to the packages folder :

Atom will load all packages from this folder by default (mine was c:\users\bas\.atom\packages). Open a command prompt and create a new symlink named after your package in this directory, pointing to your Package source. E.g.

mklink /j "typescript" "D:\REPOS\atom-typescript"

Finally restart Atom and it will show up in your packages. So you can open it from inside Atom as shown:

As one last tip Ctrl+Shift+R will reload your window + package. Good for when developing ;)