If you come from a world with a single accepted package-management system (such as Ruby with its gems), you’ll be disappointed by the lack of something similar in the Elixir world. But remember: it’s early days. There were no drive-throughs on the frontier.
At the same time, I think you’ll be pleasantly surprised at how easy it is to integrate libraries into your project once you find them.
The first port of call is http://elixir-lang.org/docs/, the Elixir documentation. Often you’ll find a built-in library that does what you want.
Next, check if any standard Erlang libraries do what you need. This isn’t a simple task. Visit http://erlang.org/doc/ and look in the left sidebar for Application Groups ...