How do i see and edited the Source Codes in Elements?
Are you trying to see the code Elements creates or are you trying to add code to Elements?
If you just want to see the code, Preview in Browser, Tools > View Source Code.
If you’re trying to add code, that’s done in Project Settings > Template.
You can also add html/javascript to your site to using a Custom Component.
If you could clarify what you’re looking to do we can point you in the right direction 