You're seeing this error when you preview, because the extension library is not installed into the Domino Designer web preview server.
com.ibm.xsp.FacesExceptionEx: Cannot find the library com.ibm.xsp.extlib.library, required by the application /Samples/xpages/XPagesExt.nsf.
com.ibm.xsp.page.compiled.CompiledPageDriver.verifyLibraryDepends(CompiledPageDriver.java:156)
Copy the library plugins and feature from where they are installed into the Designer
Notes\Data\workspace\applications\eclipse\
[the workspace used by the running Designer instance]
to
Notes\Data\domino\workspace\applications\eclipse
[the workspace used by the Designer web preview server]
Then restart Designer.