dictionary locations

I'm having a problem like Shawn Riordan's. I'm on Mac OS X 10.5.8. Aspell  
is installed and works correctly through Aquamacs. I want to use the C API  
to use Aspell in a little programming puzzle (Euler Project Problem 59).  
I've installed the en_US dictionary, and aspell works correctly from the  
command line. But when I run this code (basically exactly from the Aspell  
C API doc):


   AspellConfig * spell_config = new_aspell_config();
   aspell_config_replace(spell_config, "lang", "en_US");
   AspellCanHaveError * possible_err = new_aspell_speller(spell_config);
   AspellSpeller * spell_checker = 0;
   if (aspell_error_number(possible_err) != 0) {
     printf ("%s\nl", aspell_error_message(possible_err));
     return EXIT_FAILURE;
   }
   else
     spell_checker = to_aspell_speller(possible_err);

I get


No word lists can be found for the language "en_US".