Here's what it looks like:
You'll then need to download it and save it as your robots.txt file. (Be advised that you'll probably overwrite your current robots.txt file if you already have one.)
For now, forum response is supportive of the tool, but the big wish is that wildcards be supported in the future.
Danny Sullivan has written more about the tool at Search Engine Land, where he walks you through the process and explains the syntax of a basic robots.txt file.
Forum discussion continues at WebmasterWorld.