Well, first of all, you can't log in without registering. If you did register however, you must first activate your account. I will walk you through this:

!. Go to your E-Mail account, in this case it will be myemail@website.com
2. Then click an E-Mail entitled something along the lines of 'Welcome to Shadow Pokemon Alliance' forums
3. You should get a message that reads something like:

Welcome to Hackers League forums

Please keep this email in your savebox. Your account informations are
as follow:

----------------------------
Username: My Username
Password: My Password
----------------------------

Your account is currently inactive. You cannot use it until you visit
the following link:

http://clickablelinkthatleadstotheforums.com

Please do not forget your password as it has been encrypted in our
database : we cannot retrieve it for you. However, if you forget your
password, you can request a new one which will be activated in the same way
as this account.

Thank you for registering.

--
Hackers League

4. Then you should click the link and you will be sent to the forum page and all you need to do is log in