Are you using Post Processing Stack version 2 by any chance (it seems so)? Slate right now only supports Post Processing Stack version 1.
Using Post Processing Stack version 1, you simply need to add the “PostProcessingBehaviour” component on the “Render Camera” and attach to that PostProcessingBehaviour” a “PostProcessingProfile” within which profile you have “Depth Of Field” enabled.
I have also replied to your other post, please let me know there regarding that one.