GWT's Rich Snippet Data testing tool is quite awesome. When you test an URL, it shows you how the rich snippet on the SERP may look like. However as many answers already submitted say, that it takes some time for this to actually start reflecting in the SERP.
I don't think that's your problem though. If you are not able to see the review snippet in the tool preview itself, then most likely there is some problem that you are facing. GWT also tells you what kind of schemas it found on that page, and whether that schema is incomplete or not.
I have not worked with WooCommerce so I do not know the granularities of the way it uses schemas, but if you compare your mark-up with the syntax provided in schema.org/Review and the one in schema.org/Product then it might help.