I'm able to get out this problem.
I'd one month old installation of designer and workbench. During generating formguide, this set up created mxml with following component for Optional paragraph:
Then I uninstalled the workbench and designer and installed latest 8.2.1_GM version of those and run the formguide again. This time, the produced mxml has the following entry
(Notice the difference from cm.* to cm.controls.*)
As I guess (just a guess),I think earlier it searched CmOptionalParagraph in cm root
(it gave error that referenced component cm.CmOptionalParagraph not found as it was in cm.controls location.)
Now it is able to find that as it searches correct location.