GT Post Approval adds "Approve/Reject" buttons visible just for administrators and editors in the Menage > Posts page.
Maybe you are using the version 0.3 and you have set the option to don't display not approved post in a single post page. Take a look to Options > Approval ;)
Make me it know with a comment on the plugin homepage or in the plugin directory on WordPress.org and I will try to fix it.
Please, specify the plugin version and what expected feature don't works (for example: "not approved post are visible in every section of my blog" -- a reply could be: "do you have already checked the options on Options > Approval on administration panel?")