When I started using A5 recently I found I had difficulty loading stuff as well. Then I learned that A5 has a path-handling system using ALLEGRO_PATH objects. That seemed to solve my problems.
You first create an ALLEGRO_PATH object, then use al_get_standard_path() to get the full path of where your executable is, then you can use al_append_path_component() to go deeper into a directory structure and al_set_path_filename() to set the filename.
Then, to load a file, use al_path_cstr() to convert the contents of an ALLEGRO_PATH object into a string.
Even if this seems overly complicated, it would be good to learn because you can use al_get_standard_path() to get other path locations suitable for storing global data, accessible by all users, or current-user-specific data.