To use the built-in Code Editor in the app, you need to find the "Design/Code" toggle at the top, and click on CODE.
Keep in mind that since your animation is simply code, you can also edit its file in any editor of your choice (e.g. Atom, Sublime Text, etc.)!
Using custom code editors with Animator
The file you are looking for is located at:
Remember to replace <yourusername> and <yourprojectname> accordingly.
You can also access this file easily through the app if you have your project open by clicking on
Developer > Open in Finder or
Developer > Open in Terminal from the top menu.