It seems ST-Links SpatialKit is no longer supported or available. If this is true, could I suggest that anyone interested takes a look instead at GISquirrel www.gisquirrel.com
It provides editing of postgreSQL and SQL Server data in ArcMap, is now compatible with ArcGIS 10.6, and is fully supported. We are even looking at integration with ArcGIS Pro and lots of other nice enhancements in future.
I would not have posted in this group if this product didn't seem properly dead... if it is still breathing, then I apologise!