How to set up path for library and include files
If it's a theme function, guessing from the file name you mentioned, you should use them theme() function to get the themed output. 

To get the path of a module relative to the Drupal root, use drupal_get_path() function. 

If you want to include or require a file, you can use module_load_include(). However, note that if the including files print anything, they will be printed to the current output buffer (which means they will be printed on user browser). 

Given the above available features, I think it's theme() that you are looking for.