When you're on the page of an actual product in the store, about halfway down there are two tabs. The first is "Product Reviews" and the second is "Product Discussion." Make sure the "Product Reviews" tab is open.
The top of the tab reads "Average Product Rating: <number of stars> (based on # ratings)."
Just under that, before the first review posted starts (assuming there is one) should be a link that reads "Write a review of <insert product name here>!"
Hope this helps.