Have you set your password yet? If not, just go to https://skydeck.com/forgot_password and enter the email address you registered with. An email with a link to create a password will be sent to you. If you do not get the email, make sure to check your spam folder.
If you get a message saying that your email address is not recognized you may have mistyped your email address when registering. Unfortunately there is no easy way to recover from a mistyped email address. The best way to recover is to set up a new account by opening Skydeck and selecting ‘Stop Syncing’ then select ‘Start Syncing’ and signup with your correct email address. All your contacts will be backed up again and you will be able to log into Skydeck.com after setting a password.