Hi,
In word 2013 you are able to digitally sign the documents at the following location:
File - Info - Second button "Protect Document" - Last option "Add a digital signiture".
I believe that's the option you are looking for. It might also be that during the upgrade the Add in to sign documents sigitally was not migrated. In that case look under File - Options - Trust Center - Trust Center Settings - Trusted Publishers.
However i think you are looking for the first option.
Maurice