To install, copy the .plist file to the following directory
Username/Library/Application Support/TextWrangler/Language Modules
I also recommend going into TextWrangler’s preferences and changing the color for comments. The default is a gray color that doesn’t provide a lot of contrast compared to normal black screen text.
Also, check out the via page from SciViews.com. It has links to R syntax highlighting files for many editors on many platforms.