You could use our Integration feature to send the order data to a custom script on your system -- so e.g. you could register the buyer's email and transaction ID as their initial username and password in your system:
http://www.e-junkie.com/ej/help.integration.htm
You could customize your thank-you (download) page to place a link to your forum login or registration page, and use our thank-you page template variables to show the buyer the login/password they should use, like so:
Login: [%payer_email%]
Password: [%txn_id%]
Thank-you page customization is fully documented here:
http://www.e-junkie.com/ej/help.custom.thankyou-page.htm
As another alternative, you could simply use Integration to submit the buyer's email (and other data to use or ignore as you wish), to your own forum registration script that would automatically email them a validation link and/or randomly-generated password. You could also consider using our Send Generated Codes feature:
http://www.e-junkie.com/ej/help.keygen.htm