On commence par télécharger le module en question :
drush dl superfish drush en superfish
The following extensions will be enabled: superfish Do you really want to continue? (y/n): y superfish was enabled successfully. [ok] Superfish library is missing. Please refer to the plugin documentation for how you can fix this issue; Once[warning] done, go to the Block layout page and use any of the "Place block" buttons to create a Superfish block. If there was no Superfish block in the "Place block" form, go to the Performance page and clear the cache [status] once.
Comme le dit la procédure d'installation, il faut installer la librairie Superfish dispo ici.