This article describes how to resolve the issue of a missing decal, after the decal is applied to the part.
A common issue users find, after a decal is added to a SOLIDWORKS part, is that the decal disappears. For instance, in the image below, one can clearly see there is a decal applied to the highlighted face in the Decal PropertyManager, but the decal is not visible.
To resolve this issue, the display of decals must be turned on by going to View and enabling Decals.
Now your decal should be viewable in the graphics area.