Hi Clemilda,
Approve Assessments option would be enabled once you change the status of the Talent Review Meeting to 'Approved'. You can approve the Talent Review Meeting via the Start Presentation link.
The Performance/Potential ratings that are changed through the Talent Review Meetings will only update 7408 and 7409 Infotypes. The ratings in the appraisal/assessment documents should be given by the manager.
-Ranjith