Its currently only available in next version (beta 11).

Here's how you can access next version: http://offworldtradingcompany.gamepedia.com/Next_version

Otherwise just wait a couple of weeks till the game is released to access it.

I believe there is a small bug with the Auto-Sell Hotkey option.  I changed mine manually, but it doesn't seem to work, the only thing that works is Ctrl+Shift+ click on the sell button for a specific resource, If I use my custom hotkey and click, nothing happens.

popsursocks, I do not believe adjusting the combination of buttons + mouse to auto-sell is an option. The only option in the hotkeys section is the "toggle selected auto-sell" which is different from the ctrl+shift+click toggle and works for me just fine. Now, if you want to be able to change how ctrl, shift, alt + click work, that's something certainly worth leaving feedback about.