When you joined Hive, you chose a username and a way to login -- one of those four ways. You also should have received and copied down some cryptographic keys in a safe place. If you have the POSTING key already loaded to one of those four logins, you should be able to put your name in or scan a code. If not, you may need to set that up.
But, given that you are posting and I see your comment, you may already be logged in, but not have enough resource credits to vote right now. Wait another day, and try again.