How should I use hook_init()? I'm trying to disable the LabJS module from running on certain pages by following the maintainers instructions at Compatibility issue with Slickgrid.

This is what I have, so far. I tried putting it directly in the page and my themes template.php with the theme's name appended to the function name, but neither work.  

/**
 * Disable LabJS on certain pages
 */
function versatile_hook_init() {    
  if (current_path() == "private/members/search") {
    define('LABJS_DISABLE', TRUE);
  }
}


Can you help pointing me in the right direction?
hook_init() must be implemented in a custom module. In your example, the function mymodule_init() would go into a file named mymodule.module in a custom module named mymodule. You would also supply a simple mymodule.info file and put both files in the sites/sitename/modules/mymodule directory. 

The custom module must be enabled like any other. Also, when creating a hook implementation, it is usually necessary to clear the cache for Drupal to recognize it. Finally, note that mymodule needs to be a unique name different from other modules or themes.

As a minor usage point, current_path() returns the current URL which may not be the current alias (e.g., node/5). Using drupal_get_path_alias() is recommended to always return a friendly alias.