How to edit the discourse files? A development box?

I was now able to follow this guide:

I would like to edit and see if I was able to add some changes to: https://github.com/discourse/onebox/blob/master/lib/onebox/engine/amazon_onebox.rb

But Im not able to find the file on my mac?

Damn it have a steep learning curve this thing…

Wish me back to the old apache days… :thinking: