The Forum Access Instructions are provided on the Download Page, which you can access via the email link you received after purchasing your theme.
It shows the steps you must take to register for a Forum USER Account and get the Account upgraded.
If you have followed these instructions and still do not have access, please try clearing your cache and cookies and logging in again.
If you still need assistance after that, please use the form here http://www.studiopress.com/contact
to provide me with your forum username, the email address used for the purchase, and your Transaction ID (if known) and I will manually upgrade your account.
*PLEASE NOTE* - if you have only purchased a child theme, and do not yet own the Genesis Framework, your Transaction ID will not be recognized. Genesis is required in order for your child theme to function, and therefore support forum access depends upon a Genesis purchase.