Googler, Pierre Far, has announced on Google+ that they have produced a “troubleshooting guide” for the new data highlighter tool.

This guide is available over here and contains frequently asked questions on issues one may stumble upon when using the new tool.

Here is the current set up information on that page:

Creating a page set:
After I enter my starting URL and click OK, I get a “URL not found” error.
When I try to create a custom page set, an error message displays.
Data Highlighter doesn’t understand my site.

Publishing a page set:
Extracted data displays on the Structured Data page, but still no rich snippets are shown in search results.

Forum discussion at Google+.