I've installed a rich snippet extension, and everything is correct when I use the rich snippet tester. But the rich snippets don't show in Google. How to fix this?
Usually it takes some time for Google to show the rich snippets. The time is around 1 month or so depending on your website crawling rate.
There are many reasons why google doesn't take your markup data into account and.
I recommend you to double check if you follow google guidelines to markup, and then check it with Google's structured data tool - https://developers.google.com/structured-data/testing-tool/
Also, there may be issues with the way your extension manages structured data markup. Alternatively, you may check some other solutions (e.g. http://www.mageworx.com/magento-rich-snippets.html) and see how they will work for your website.