Install the Air Force Splinter folder into EITHER of those two folders:

SavedGames/Users/(You)/DCS.openbeta/Liveries/Su-27
If you dont have an Liveries and Su-27 folder, create them.

OR

(Game Install) DCS World/Bazar/Liveries/Su-27 



Install the RoughMet folder to 
(Game Install) DCS World/Textures/