Same issue with attempting to install either the original US HAZUS-MH and the new Canadian Hazus-MH: warning dialogue with full cancellation of the installation attempt, and no possibility of overriding to install anyway.
More frustratingly, it does not appear that it is possible to downgrade 10.1 to 10.0, nor to acquire an older version of ArcGIS. I've put in requests to Esri sales on how to acquire the older software, so far to no luck.
Related: Has anyone encountered problems with having ArcGIS 10.0 SP2, installing Hazus-MH, then upgrading to ArcGIS 10.1? (ie, if we successfully find a way to install, will we be stuck using outdated versions of Arc, or is it just an installation-hiccup?)