Apparently this plugin doesn't work on Xcache versions prior to 1.3 because of a dependency on the function xcache_unset_by_prefix().
Unfortunately Dreamhost offers 1.2.2 to all its users. That's a lot of people.
Ideally we'd want to replace that function with something that does the same thing but in the mean time, perhaps the plugin can automatically detect if the environment supports that and could notify the user that they need Xcache1.3
I've updated the plugin with this fix and am attaching a diff for this.