You have to take one of the skins that are already made in the F/A-18C folder, make a copy of it on your desktop and rename your new dds file that same name of the file in that folder.....Copy and paste it in the folder and when it asks you to overwrite it just say yes!
Now give the folder on your desktop a new name like "VF-113 New" or your name and then open up the description lau and change that name to the name of your new folder, save it and then drop it in the F/A18 Hornet skins folder and go in game and find it in the ME.
If if to confusing just send me your file and Ill have it done in a snap