El Dorado is a quest given by Rachael Skull. You need Firearms level 2 before you she'll consider you. Take Rachael southeast to a police station. Once you arrive Rachael will open the fence and immediately get in an argument with Bat Jacobs. Rachael locked Bat in the police station when the infection broke out and even though that saved his life he still isn't happy with her.
Rachael asks where the gold is. Bat doesn't want to tell her anything so it's up to you to persuade him. Tell him that he's outnumbered and he responds that he'll shoot Rachael first.
If you want to recruit Bat and earn the Just a Patsy awardment make sure you tell him: "I don't care about her. I'm only interested in the gold". Rachael wont like that much, but as long as you've completed some Fort Kohai quests and won some fisticuffs challenges she'll still be happy with you.
Bat then tells you that he was double-crossed by his former accomplice Kit Wakeman who drove the armored vehicle full of gold away. You now have a new dialog option to ask everyone in the world who Kit Wakeman is.
After the conversation is over and if you still plan to recruit Bat, now is the perfect time to rob Rachael and make sure Bat is watching you do it. Tell her this is a shakedown! Again she wont be happy about being robbed, but Bat will gain a huge amount of approval and respect for you. If you don't rob Rachael you'll have a very hard time recruiting Bat and earning the awardment.
Next get your payment from Rachael. If you robbed her you wont get any gold as payment, but if you didn't rob her she'll reward you with 100g.