Hello,
In order to get the mass plugins in the “Plug-ins” view, the plugins needs to be installed on the platform (meaning your Capella Studio) so by extracting the dropin version of the addon to the dropins folder or by installing the addon through the update site.
I just tried it with Capella Studio 5.2.0 and the Basic Mass viewpoint 5.1.0. It does works correctly and if I regenerate the viewpoint from the vpdsl files and restart in runtime then it still works.
Regards,
Arnaud