Where is tmp file directory?
Look in your Drupal admin area at /admin/config/media/file-system

This will tell you where Drupal thinks it's tmp folder is currently set to.

Depending on your host provider you may or may not be able to write to that location.

The php-info report found on your Status Report page admin/reports/status can tell you where PHP's temp directory is defined to be on your system:

upload_tmp_dir     no value

Using htaccess, or ini_set in settings.php you can tweak where the upload temp directory is for your Drupal installation if need be.