1) the registration email is marked by your email-program (i.e. thunderbird) as spam (or "junk"). In this case simply look into your spam folder.
2) your email server is blocking the mail. Some email services like AT&T, RR or Comcast block registration emails because they think it's spam. In this case, you can either contact your email provider ( i.e. http://att.net/blocks ) or use another free email service like Gmail, GMX etc. There's no way to manually activate an account, the password is encrypted (even for me!) and you need a working email address. I can't even contact you, because the block all mails from my server.
I regularly delete non-activated accounts and those who never logged in to free the database.