The FE CSS Builder Plugin will activate and function without an active license, but requires an active license to receive auto-updates. To activate your license simply locate your FE CSS Builder license (either through your purchase email or through your " My Account" page on CobaltApps.com) and then paste it into the provided license text field in your FE CSS Builder settings admin page located in Appearance > FE CSS Builder in your WordPress Dashboard. Then "Save Changes" and then "Activate License". If your license is in fact activate (i.e.. it hasn't expired or something like that) then you should now see the word "active" in green right next to the license key textarea.
If for some reason you are unable to activate your license and it has not yet expired then you should contact our support team through your " My Account" page so we can try and figure out what's going on.