".autocomplete-plus input.hidden-input":
  "tab": "autocomplete-plus:confirm"
  "down": "autocomplete-plus:select-next"
  "up": "autocomplete-plus:select-previous"
  "escape": "autocomplete-plus:cancel"

".editor":
  "ctrl-shift-space": "autocomplete-plus:activate"

".autocomplete-plus input.hidden-input":
  "tab": "autocomplete-plus:confirm"
  "down": "autocomplete-plus:select-next"
  "ctrl-n": "autocomplete-plus:select-next"
  "up": "autocomplete-plus:select-previous"
  "ctrl-p": "autocomplete-plus:select-previous"
  "escape": "autocomplete-plus:cancel"

".editor":
  "ctrl-shift-space": "autocomplete-plus:activate"

Add ctrl-n,p emacs bindings for next, previous
Add ctrl-n,p emacs bindings for next, previous

Emacs binds are supported through most of the rest of Atom. Plus it's rough having to drop down to the arrow keys in the midst of typing :)