the tattoo should be on the right cheek (the right for the protagonist) and dark red. In addition, you should put on a desert headwrap (sold in one of the shops in Sunshine). But the main thing is that you need a map leading to the slavers' hideout. The map can be obtained by beating up an entire caravan, or by entering through a secret passage into the "House of Slaves" in Sunshine. The secret passage is in the northeastern corner of the wall surrounding Sunshine. If the protagonist is disguised (that is, he has a headwrap on his head and a tattoo on his face), you can simply take the map from the House of Slaves. Without disguise, you will have to stun the slaver.
Having received the map, you can go straight from the secret entrance to the slavers' hideout. It is better to avoid meeting with caravans. In the slavers' hideout, you will have to bluff the guard guarding the entrance (it is advisable to have a high level of SPE). In the slavers' hideout you'll find a compass, and then meet Cassie and her old friend Michael. He'll direct you to the gold mine, where there'll be a lot of fighting.
As for Nymosa, it's in a completely different place, in the inner Forest Park. The entrance there is blocked by a giant plant.