Unzip to folder FA-18C_hornet
Path:
Saved Games/DCS.openbeta/Liveries/FA-18C_hornet
If you don't have a Liveries folder, then you have to create it and create
FA-18C_hornet folder within the Liveries folder.

If you have a user created skin, then place the file "FPU_8A" in the folder of
thet created skin.

If the same file already exists, overwrite it.

If the same file already exists, but with different name (e.g."FPU_8A_VMFA-312")
then copy the name of that file and delete it. 
Then rename the FPU_8A file to the name of the previosly deleted file.

If there was no "FPU_8A" file or file with different name (e.g."FPU_8A_VMFA-312")
then insert the following lines into description.lua file.

Open description.lua with Notepad++ and insert 
{"FPU_8A", 0 ,"FPU_8A",false};
{"FPU_8A","FPU_8A_Diff_RoughMet",false};
Dont forget to save it.