Hello -
You can sign up on the Windows Marketplace for Mobile site here: http://developer.windowsphone.com/Signup-Create-Account.aspx
When it asks you to sign in using your Windows Live ID, make sure you sign in with the Live ID that your DreamSpark account is verified with.
I hope this helps, please let me know if you have any questions or run into any trouble.
Regards,
Neil Carter
DreamSpark Support