If you have not done a search you need to do that first, after you done that it takes around one work day for the candidates to be added to your campaign. If more time than that passed, reach out to your customer success manager.
On the other hand, if you done a search and contacted all candidates in your review list and no new ones show up, it means that the candidate pool for your last search is out. In that case you need to go to the search tab and create a new search.