You have to create a certificate in the master db first.

I used a self signed cert example:

USE MASTER;

CREATE CERTIFICATE testlogincert

ENCRYPTION BY PASSWORD = 'pounlmdsf'

WITH SUBJECT = 'your login',

EXPIRY_DATE = '12/31/2012';

GO

And then map a user to it.

CREATE LOGIN 'your login' FROM CERTIFICATE testlogincert;

GO