Drashna, yes, I've installed the WHS2011 version (.wssx extension). When I open the Essentials R2 dashboard, it registers under the Applications tab as being installed. However, if I launch the dashboard in safe mode as suggested, I'm prompted with a list of availabe Add-ins. Stablebit Scanner is not among those listed. It worked fine in Essentials, but so far not in R2. Would prefer to use the add-in than installing the full version if possible. Thanks!
EDIT: 2/25/14 - for others who may be experiencing this same issue under WS2012E-R2 OS, support just provided me a new BETA build of the .wssx that specifically addressed my issue. Now the expected StableBit Scanner tab is available via the server Dashboard.