I'm not sure if this helps you or not, but I copied my zimlet into the username\Documents\zdesktop\zimlets folder, and then restarted Zimbra Desktop (not sure if this is accurate for ZCS, as I'm running ZD on Ubuntu at the present time). The first time I started Zimbra Desktop, it seemed to take forever. Then after that, the zimlet worked (it was the RSS Feeds zimlet, to be specific).
Hope this helps, and have a great day

Patrick.