According to the Dr. Horrible Twitter, they’re working on fixing the setup so it’s available internationally – in the meantime, you could try this.
[Update: They did fix the international issue, but then a while later the whole site crashed, which is where we are now. Those of you who can get it through iTunes might as well - two dollars for the first act or four for a season pass - cheap, and more reliable/higher quality than watching through the site!]