How do I activate voice dial outside the U.S.?
Try long-pressing the Search button.  It should be tied to Voice Search by default.  Then say "dial xxx", where "xxx" is either a phone number (with the area code) or name of a contact from your phonebook.

Here are all the voice actions you can perform with the Voice Search app: http://www.google.com/mobile/voice-actions/

Note:
According to Google's last blog post on the subject, voice actions via this app only work in English language.  You might need to change your phone's locale to "US" or "UK" in order to get this to work.