Firstly, stroll on over to the Arcadia github page and follow the instructions for setting up your Unity project to use Arcadia. The simplest method is to clone the project into your Assets folder, i.e.
cd path/to/unity/project/Assets
git clone https://github.com/arcadia-unity/Arcadia.git
After you've cloned Arcadia, make sure Unity is running in the background and uses the full .NET API.
After you've restarted Unity, you should see in the console that the Arcadia REPL has started. You then have a few choices in terms of editor:
- Emacs