Registry Power Cleaner is simply sold by McAfee under license from Winferno. Support for it would be through Winferno's site: http://www.winferno.com/support.aspx
However if you aren't pleased with it there is a 30-day money back guarantee.
The link for refund information is http://www.winferno.com/refund_policy.aspx
If Registry Cleaner was purchased with a credit card Winferno should be contacted before any contact with the credit card company. If a dissatisfied customer doesn't do this Winferno will invoice them for "all fees and costs associated with resolving their disputed charge."
I have to say that there have been some complaints about Registry Power Cleaner, including one made in a thread in these forums, and at least one of the SiteAdvisor Experienced Reviewers classified it in 2009 as rogue software.
In general, I follow the Microsoft line on registry cleaners -