It was some good discussion about it so read this page (by few posts) about review for the signal you subscribed to: Signals : Ask & Say Anything & Everything About MQL5 Trading Signals
You can only leave a review for a signal if you are a valid subscriber. If you have been a subscriber in the past and now you have unsubscribed you cannot.
If you are a valid subscriber, just got to the signal page, click on the review tab and you will see a + review icon on the far right of the page. Click that and you can write your review.