I use a Mac as well. You can set TextEdit to ‘plain text’ instead of ‘rich text’ and make sure it’s saving as a UTF-8 .txt file in the preferences, but there isn’t an easy way to show the line numbers. If it’s saving as .rtf, that might be one reason your code isn’t working for you currently.
As for a Mac text editor, I would recommend TextMate, which is a free download. I use it for some of my bookkeeping work; I prefer to avoid TextEdit. It’s very possible that some of the more code-savvy users here might be able to recommend something better.
For ChoiceScript, I currently use @CJW’s CSIDE (which is what that screenshot was from). Unfortunately, the public version isn’t out quite yet. It looks like the public release will be soon, though, which is exciting! Meanwhile, maybe try TextMate?