If you were a member of Aussiecon IV (last year's Worldcon) or Renovation (this year's Worldcon) as of January 31, 2011, you are entitled to nominate.
Information about the process is available here:
The online nomination form is here:
If you have lost the email with your nomination PIN you still have time to request a new one. Information on doing so is available at the nomination form page.
And yes, Yipe! The Costume Fanzine of Record IS eligible for nomination for Best Fanzine this year. If you enjoy our 'zine, I hope you will include it for consideration while making your nominations.