To display the list of available candidates, go to Candidates and click the green icon next to the heart icon visible above the candidate list:
The list will now only display candidates who:
are not involved in any recruitment process;
are involved in one or more recruitment processes, but are not on the New or Hired stage (or however you named it in your recruitment process).
If you click on the green icon again, it will disable the filter, and the list will display all the candidates in your database.