User Synchronization does not work

I have just installed the User Synchronization plugin on two single site localhost development sites, and attempted to sync between them. No go: it reported a "connection error" and that I should check my URLs and Security Key. I did all that: still no go. I re-did everything (several times): still no go.

So then I installed the plugin on a live site (as master) and attempted to sync to a localhost slave. This time it reported a successful sync, but no users were actually synced at all.

What suggestions do you have?