When you register an account on BiggerPockets.com, the site automatically sends out an email to confirm you are who you say you are. In order to login for the first time, you must click the activation link in that email.
This message is sent out within a minute or two of your registration, so if you don't get it right away, chances are something is wrong.
Check your SPAM email to see if the activation message ended up there. If it didn't and you don't get the message, click the contact us link at the bottom of any page and let us know you need to have your account activated. Be sure to include your USERNAME in the email, and send it from the same account that you registered with, so we can quickly find and confirm the BiggerPockets account.
Finally, to make sure you get all system emails, add admin [at] biggerpockets.com AND webmaster [at] biggerpockets.com to your safe list.