Hello,
The Enterprise Management Solution (EMS) is the highest edition of Kentico
CMS with all features and modules enabled. So, if you had only a trial key (time limited) of the EMS edition and after it has expired, you replaced it with a free edition license key, some features and modules won't be available. For a further comparison of the free edition and the EMS edition please see this file:
Kentico 6 editions.
Anyway, the workflow module is not available in the free edition.
Best regards,
Michal Legen