What's the URL of the page containing the problem?
http://www.reviewboard.org/docs/manual/dev/admin/management/repositories/#determining-repository-information
Describe the problem and any recommendations below.
There is no information on setting up Mercurial repos. There should be, particularly about whether or not http can be used to access a repo -- currently it appears broken in 1.5 beta 2, but the only indication of this is diffs failing to apply to the raw HTML output of the mercurial repo.