Getting permission error when installing CiviCRM under Drupal
I faced this problem too and tried a lot of things to make it works, but failed.

Finally, I opened the file "index.php" in "http://(mysite)/sites/all/modules/civicrm/install/index.php". I put the comments in line number 265th through 274th like this.


/*
  // Check that user is logged in and has administrative permissions
  // This is necessary because the script exposes the database settings in the form and these could be viewed by unauthorised users
  if ((!function_exists('user_access')) || (!user_access('administer site configuration'))) {
    $errorTitle = ts("You don't have permission to access this page");
    $errorMsg = ts("The installer can only be run by a user with the permission to administer site configuration.");
    errorDisplayPage($errorTitle, $errorMsg);
    exit();
  }
*/


Yeah. IT WORKS!!!.

Who use this way like me, DON'T for get to remove the comments after installation is completed.