You have to do three things to be able to activate.
First, you need to clear your cache and cookies. This is very important, as the cookies have been changed. If the cache and cookies are not cleared, you can't get to the next step.
Secondly, you need to reset your password, as your password is less than 8 characters long.
Thirdly, you need to activate.
Be sure to carefully read the instructions you've been linked to for the first and last steps. For the cache and cookie clearing, each browser is different, and has different requirements to be able to successfully do this. For the activation, if you don't follow the directions exactly, this step will fail.
Finally, if the email address you've registered with in the archive is no longer active, before even doing any of the above, I'll have to change it for you. I have people email me at technicalsupport@adult-fanfiction.org with the email address they want to have me replace the old inactive email address with.