The email is sent automatically when you register an account at pbreview.com. If you do not receive the email it was probably swept away by an overzealous spam filter.
There are a few different solutions for this problem:
1) Check your spam or junk mail folder. The activation email may have just been moved there.
2) Temporarily disable your spam filter and have the activation email sent again (see below).
3) Add "firstname.lastname@example.org" to your address book and have the activation email sent again (see below).
4) Add "pbreview.com" to your trusted or accepted domain names and have the activation email sent again (see below). How exactly you do this varies significantly with different email and spam programs, but most give you the ability to do this in some way or another.
If you want to have the activation email sent again, log in to your inactive account and click the "Email Activation Code" button that comes up. The email should come in a few minutes if your email address is accepting emails from pbreview.com.