There are breaking changes in SQL Server 2008 R2.
Download source code and comment those lines :
* Legacy Code (SQL 2005 et 2008)
//IObjectExplorerEventProvider provider = ServiceCache.GetObjectExplorer().GetService(typeof(IObjectExplorerEventProvider)) as IObjectExplorerEventProvider;
//provider.NodesRefreshed += new NodesChangedEventHandler(Provider_NodesRefreshed);
//provider.NodesAdded += new NodesChangedEventHandler(Provider_NodesRefreshed);
//provider.BufferedNodesAdded += new NodesChangedEventHandler(Provider_NodesRefreshed);
ServiceCache no longer exists in R2 and some ways to subscribe to these events found on internet do not work everytime (not on my laptop for example).
I do not know the real effects but addin works on my SQL Server 2008 R2 instance.
Note : you might need to change some references (in x86 folder if you are in 64 bits) and SMOEnum doesn't longer exist too (SMOExtended and/or SQLEnum) before rebuild.