Hi.
When the plugins are showing up under "File"->"Application"->"Application management", that means you've successfully installed the applications into the Designer tooling, so the XPages Editor will have those controls listed in the Controls palette.
You also need to install the plugins into the Web Preview server, to fix the problem:
>The application requires com.ibm.xsp.extlibx.library. This library cannot be found.
You will see the plugins installed at this location, used by the Designer tooling:
Notes\Data\workspace\applications\eclipse\plugins
Copy the plugins from there to this location, used by the Web Preview server:
Notes\Data\domino\workspace\applications\eclipse\plugins
Also, rename this folder to backup (or whatever):
Notes\Data\domino\workspace\.config\org.eclipse.osgi\
Then close Designer, wait for all the Notes and Designer related processes to finish, and when you restart Designer and open the page in Web Preview, it should find the library OK.
The information about the extra steps being needed for Web Preview is mentioned here:
http://www-10.lotus.com/ldd/ddwiki.nsf/dx/Installing_an_XPages_Library_into_Designer#Preview+in+Web+Browser