Anyone who was a member of Noreascon 4 (the 2004 World Science Fiction Convention) or who is a member of Interaction (the 2005 World Science Fiction Convention) is eligible to fill out a Nominating Ballot.
Interaction has put the Hugo ballot up on their webpage as a PDF; and they also have an online ballot you can fill out, provided you have your Hugo PIN from either Noreascon 4 or Interaction. Both of these ballots can be found at http://www.interaction.worldcon.org.uk/hugovote.htm.
Even if you can only think of one or two things you might want to nominate, consider taking the few minutes to do it anyway. The more people who participate, the more significant the results.
Note that Friday, March 11 is a "received by" date, not a "mail by" date.