One other item that mattered… After successfully getting the XPages toolbar Plugin to install successfully, when I opened my JAVA Class file that imported the toolbar, the Import class file directive was still not resolving.
It turned out that the –> 'org.openntf.xsp.debugtoolbar_1.1.0.201403092102.jar' file was NOT appearing in the JAVA Build Path as shown in the 'Properties/Java Build Path' UI in the Domino Designer Package Explorer when selecting my application NSF file.
By searching for the Xpages debug JAR file name (beginning the search in the root of the \Notes directory), I eventually found the Xpage Toolbar JAR file's location, noted the full path to the JAR, then went back to the 'Java Build Path' UI and clicked on the the 'Add External JARS…' button within the JAVA Build Path UI.
I navigated to the JAR, selected it, clicked the 'Apply' button and closed the Java Build Path UI dialog and waited a couple of seconds.
The Designer client immediately updated and resolved my error appearing on my Import. It could now resolve the XPage Toolbar class.
This detail is for those who might suffer the same pain that I went through to resolve this.
I'm still not sure what happened during our in-place upgrade from Notes 9.0.1FP9 to Notes 11.0.1, but it clobbered this class file along with mucking up my Designer 'Preferences/Plug-in Development/Target Platform/Target definition choices.
The XPages Toolbar Plug-in has given me years of non-stop service up through Notes 9.0.1FP9. Only when we performed an in-place upgrade to 11.0.1, did it fail.
Thank you Mark for a great tool and continued support!