I struggled with this too. Install the skins/liveries in your Saved games folder for DCS not the actual game folder. If you have not installed liveries before you will need to create the Liveries folder.
"C:\Users\<username>\Saved Games\DCS\Liveries" (my Saved Games folder is on C:\ drive).
Make certain that you create the Liveries\<module> folder (in your Saved Games DCS folder) using the exact same folder name used in the
D:\DCS World\CoreMods\aircraft\<module folder>\Liveries\ folder. (D drive in your case).
For example, the Liveries folder for the HeatBlur F-14 has two sub folders: F-14A-135-GR and f-14b. Note the inconsistent folder naming convention used. Note also that the F-18 Liveries folder has three sub folders! The FA-18C_hornet is the one for the purchased module.
I would not use OVGME if I didn't have too. I recommend that one use a few add-ins like that has possible. Besides, in the case of Liveries it's super easy to install them manually once you know how.
Start/Restart the game after you have installed the Liveries.