Installation is done using Atom package manager or command
apm install autocomplete-erlang
CAUTION: MAKE SURE TO HAVE
autocomplete-plus PACKAGE INSTALLED
It seems that OSX has a lot of different safe measures which don't cooperate nicely with atom environment.
For optimal behaviour always start atom from commandline instead of Finder.
Let me guess. You're using OSX. This happens when starting atom from Finder. Finder-started applications have no access to PATH variable. To go around that make sure to set "Elixir Path" in package configuration to Your absolute elixir executable path or start atom from command line instead.
Good catch. Let us know what about this package looks wrong to you, and we'll investigate right away.