You can add a comment for a candidate by clicking the respective button at the top of the Candidate view, as shown below:
You need to fill the following information:
Comment: Optionally write a comment about the evaluation.
Evaluation: Click on one of the stars (1-5).
Click on the Submit button to add the evaluation, which will immediately appear in the candidate's timeline.
The evaluation average will appear at the right of the candidate's name at the Candidate view, as shown below: