Text in areas (and cutscenes which are technically the same as areas) is found inside the Map.bin file inside the area subfolder; those are binary files, compressed with GZip, with ASCII-encoded text in them.
As for how to edit the text, I don't have the editor with me at the moment, and I don't remember how it works. I'll have a look at it tomorrow if nobody else answers your question.
EDIT: select the screen where you want to add text, then click on the Screen Settings button; in the [General] section you can assign the text you want (on a single line) to the Header property. You can have a look at the code in Lomond as an example.