Reader Extension is actually a Adobe LiveCycle Server module, it is not LiveCycle Designer plugin. For using that you have to install the LiveCycle ES4 Server with Reader Extension module.
You could also check this feature of Acrobat : http://help.adobe.com/en_US/acrobat/X/pro/using/WS58a04a822e3e50102bd615109794195ff-7e0d.w .html
-Wasil