If the mod doesn't show up in the launcher: Once the download has completed, tick the checkbox to activate the mod.At the same time a 'Crusader Kings II - Workshop Content' download will be triggered in Steam client (where you can monitor progress).Under 'Mods' tab, the new mod will appear at the bottom of the mod list as unselectable, with a red background (which does not reflect download progress !).Find the mod on the CKII Steam Workshop.

mod file already exists with same name under ~\Documents\Paradox Interactive\Crusader Kings II\mod\ it will conflict during the installation.