In your image above I see a mane on Whisper. Select this and apply the Gold mane .mc6, long or short whichever you prefer TO this mane that you have selected. The color change may not be visible so render and see how it looks.

The gold mane will not show up under characters, an .mc6 file is not a character file, it just changes the textures/colors/transparencies of the character you have already selected (the mane). You need to render to see it sometimes.