To edit plain text files on the Mac requires using a plain text editor. The TextEdit editor included with MacOS only works if you follow these instructions: http://support.apple.com/kb/TA20406. There are a few alternatives, though. The best lightweight tool is a cross-platform text editor called: Sublime Text; just Google it and you'll find a download site or go through the AppStore. Other editors include AFAIK, TextWrangler, and BBEdit. Here are some download URLs:
http://www.barebones.com/products/textwrangler/download.html
http://www.sublimetext.com/3
http://www.barebones.com/products/textwrangler/download.html
http://www.sublimetext.com/3