What do you mean won't show up? Do you mean you can see your marking on the list but nothing happens when you click it? Or do you mean nothing is there at all?
Make sure your image is:
- One layer
- .png format
- Saved in your Textures folder.
With the markings.cfg file:
[Fbody]
Cheetah;fmarkCheetahBody.png
Cheetah 2;fmarkCheetah2Body.png
Cub Spots;fmarkCubSpotsBody.png
Back Splotch;fmarkBackSplotchBody.png
Jaguar;fmarkJaguarBody.png
Jaguar2;fmarkJaguar2Body.png
King Cheetah;fmarkKingCheetahBody.png
King Cheetah 2;fmarkKingcheetah2Body.png
Leopard;fmarkLeopardBody.png
Leopard2;fmarkLeopard2Body.png
Leopon;fmarkLeoponBody.png
Liger;fmarkLigerBody.png
Marble;fmarkMarbleBody.png
Marble2;fmarkMarble2Body.png
Ocelot;fmarkOcelotBody.png
Scar;fmarkScarBody.png
Snow Leopard;fmarkSnowLeopardBody.png
Swirl;fmarkSwirlBody.png
Tiger;fmarkTigerBody.png
Tiger 2;fmarkTiger2Body.png
Siamese;fmarkSiameseBody.png
Bubbles;fmarkBubblesBody.png
Spiral;fmarkSpiralBody.png
Underfur;fmarkUnderfurBody.png
Stripes;fmarkStripesBody.png
Stripe Spots;fmarkStripes2Body.png
Collar;fmarkCollarBody.png
Subtle Spots;fmarkSubtleSpotsBody.png
BackLeg Stripes;fmarkbacklegstripesBody.png
Striped Legs;fmarkStripedlegsBody.png
Butt Stripes;fmarkbuttstripesBody.png
Dorsal Stripe;fmarkdorsalstripeBody.png
Toeless Socks;fmarktoelesssocksBody.png
Back Stripe;fmarkBackStripeBody.png
Splatter Back;fmarkSplatterBackBody.png
Splatter Socks;fmarkSplatterLegsBody.png
Splatter Full;fmarkSplatterFullBody.png
Chimera 1;fmarkChimeraBody.png
Chimera 2;fmarkChimera2Body.png
Celtic;fmarkCelticBody.png
Butterfly;fmarkButterflyBody.png
DiamondBack;fmarkDiamondBackBody.png
Dalmatian;fmarkDalmatianBody.png
Fading Spots;fmarkFadingSpotsBody.png
Fading Stripes;fmarkFadingStripesBody.png
Hellion;fmarkHellionBody.png
Copy and paste one of them
Hellion;fmarkHellionBody.png
The display name of the marking.
The image name and format.
Go File>> Save to save the markings.cfg file after editing.
If you're using Preset Maker then make sure you clicked "Custom" on the last tab, that way any marking you choose will show.