Testing a app script

I have created a script via Drive|New|App Script

Now I want to test it. This must be so fundamental that it must be obvious, but it is not to me.

My script is for a Document. I used the code example from this "Translate" example, http://ift.tt/N3Y1YM

I have tried creating a new document and then from the menu Tools|Script-editor I have gone looking for my saved project, but the "Open recent projects" is empty.

How do you test/run a saved app script project for a Apps Document?

Thank you in advance.