The "Add to canvas" menu is a pop-up that is displayed when you right-click on the raster table from the DB Manager Menu (Database => DB Manager => DB Manager)
I have an out-db raster on PostGIS which I tried to load in QGIS (v 2.14.12-Essen) utilizing the DB Manager to establish a connection. I could view the raster table, but when I try to "Add to canvas" it displays a black screen instead of the raster image.
There is data in the table?
I am not that familiar with GIS, much less QGIS. Still a quick look at the docs found this: