...or OpenID (according to the page itself).
You can opt to associate your Stack Overflow account with your CV, and it will appear on both your public and private CV views.
All account matching is by OpenID. For best results, please sign in to careers with the same OpenID that you use on Stack Overflow.
How this exactly works, I am not 100% sure as I remember someone here saying that there is a problem with Google assigning unique IDs with every login - I am guessing that they either found a workaround or Google no longer does this... however, I am no expert and really can't comment here.