When you create an account with Chain you will get an email to verify your account.
It looks like this:
The link expires after 24 hours, but if it does don't worry, it is very easy to get another!
Just return to the Chain toolbar, click any icon to open the login screen and enter your email in the login dialog:
Once you do this it will send you another verification email! Click 'Verify your email' and then you should be able to login to Chain.