Once the extension has been activated, you must rebuild your index on the Advanced tab of the SearchWP settings screen, and then trigger a reindex.
While the index is rebuilding, youll notice a number of new Custom Fields in the SearchWP settings dropdown for each post type, they all start with coauthorplus_
By default the extension will index:
and there are separate entries for each of those, but the extension also bundles all of those fields into a coauthorsplus_any Custom Field if you want to match any of those.
Once the index has rebuilt you will be able to search by Author data as defined by Co-Authors Plus. Searching for configured Author data will return the post(s) written by that author, not the Author archive or a User profile.