Once you have registered, you will be sent an email containing a link. This will be sent to the email address you have used to register. To activate your account simply click on the link in the email. This confirms that the email address is correct and genuine. Once you account is activated, you are free to search and send messages to other members.
If you don't receive an email asking you to activate your account, then you can request it again by clicking the "request new activation email" from the left hand side of the page once you are logged in. You are still able to log in even if your account hasn't been activated. Remember to check your email spam/junk folder just in case the activation email has been automatically filed there.